and filling the spectator with mingled emotions of delight, of awe, and of terror. But this is not half. The whole of the surrounding belt, from its periphery at the base of the great walls of Kilauea, to the elevated central platform-and over eight miles in circumference by half a mile in diameter, is in a state of intense activity. Over this surface I could count sixty lakes of fusion whose flaming fires were sparkling, and surging, and dashing, and leaping in the most fantastic and brilliant manner, exceeding all that pen or pencil can paint. The whole of this surface is not, of course, broken and fused at once; but it is every where rent with fissures, studded with burning cones, and dotted with boiling lakes and pools of igneous fusion; and even the solid portions of the surface are so hot as almost to crisp the scle of one's shoes, while the smoke and the pungent gases render it difficult to travel in some parts and impossible in others. At the terminus of the winding trail by which you descend into the crater, a great lake of fire has been opened, so that a party which visited Kilauea some three months ago, returned without effecting an entrance into the crater.

During the last week in May and the first in June, visitors and passing travelers reported a fiery girdle around the whole circumference of Kilauea, along the base of her lofty walls-and, so intense was the heat, so suffocating the gases, so fearful the hissings, so awful the surgings, and so startling the detonations, that horses wheeled and plunged with panic, and men retired from the old Kau and Hilo road which, as you may recollect, lay near the upper precipice, and passed the great fissure at a respectful distance. And I have been told by those who observed and felt it, that so great was the heat on the road above the western precipice, 700 feet above the fires, that they were obliged to hold their hats between their faces and the crater, and pass rapidly along to avoid it. The upper banks also of the crater are smoking and steaming intensely, and sometimes the vast cauldron of Kilauea is so filled with dense masses of steam and smoke as to obscure all the glowing fires below, and to represent by a fact which mocks all fable, those Plutonic realms where fire and darkness forever reign.

Again when the winds are free and the fires more active, this suffocating mass of heated gases, vapor and smoke rolls off, and leaves the abyss below, all radiant with ten thousand brilliants, while the sparkling and dancing fires shine up gloriously upon the heavens, forming a lurid pillar whose brilliance is seen along the shores of Hilo, Puna, and Kau, and far off at sea.

For twenty years I have watched the movements of this great crucible of nature,-this Hawaiian volcano-with intense interest, and never perhaps, have I seen the fires more extensively distributed over the crater or more active and vivid in their play.

We may be entertained with another grand eruption; but when and where it will burst the adamantine walls which now confine the molten seas, we know not.

I said that I had lately visited Kilauea. During the latter part of June, I left Honolulu where our mission had held its annual meeting, in company with a party of ladies, gentlemen and children, bound to Hilo, via Kau and Kilauea. We reached the volcano on the third of July, and left in on the fourth. But I regret to say, that, in consequence of a sprained foot, I was unable to descend into the crater. I could therefore only survey it from the upper banks, and receive the reports of a party who went down to the fires.

At Kilauea I was met by my son, T. Munson, from Hilo, with an instrument to measure the angle of inclination of the stream of lava on the northern precipice of the crater. On the fourth of

July he went down for the purpose; but as all that portion of the crater was full of boiling cauldrons, and as the whole bank was enveloped in dense smoke and deadly gases, he was unable to approach it. Meanwhile I had proceeded on toward Hilo with the ladies and children, none of whom could be persuaded to go down into that fiery abyss on account of the fearful activity of the fusion.

I exceedingly regret this failure, and the more so as I know not when I shall be able to visit the spot again. Should I be permitted however, to revisit the scene, nothing but an impossibility shall prevent me from attending to your request, and sending you the angle of that slope.* I am ashamed that it has not been done before, and my apology is, as stated, want of opportunity, unless I had gone up on purpose. Meanwhile, you will I think, rest assured that that angle is not less than 40. As I have measured other slopes, I compared them in my mind to what I recollect of that when with extreme difficulty I clambered up it in 1835. I intend however, to give it to you correctly, if spared to visit Kilauea again.

You ask if there were any small cones thrown up along the course of the eruption of 1852. There were a few.

In the writer's Exploring Expedition Report on Geology, p. 179, these streams of solid lava descending the enclosing slopes of Kilauea are mentioned. Mr. Coan writes in reply to a request that he should measure the angle of declivity, there being some doubt with respect to it.The above account by Mr. Coan will be better understood after a reference to the sketch of Kilauea in volume ix, of this Jour. nal, p. 352. The great dome Halemaumau corresponds to the lake, a, in the southwest extremity of the crater.-J. D. Dana.

ART. XIII.—On the Aperture of Object Glasses; by F. H. WENHAM, republished from the London Quarterly Journal of Microscopic Science for October, 1855.

PROFESSOR BAILEY having noticed in the July number my remarks bearing reference to the fact of his being able to discover the markings on the most difficult tests known, when mounted in balsam, I beg to state, that my observations were dictated by no other motive than the desire of establishing a correct fact, and that I was not prejudiced by any favorite theory.

Professor Bailey says, 'It is apparent from the above that Mr. Wenham has convinced himself, both by reason and experiment that I ought not to have seen the markings on delicate test-objects when mounted in balsam.' From this I infer that Prof. Bailey had not seen a paragraph contained in my communication in the 'Quarterly Journal of Miscroscopic Science' for January, 1855, page 162, or I feel assured that he would not have thought it necessary to make this form of reply, for I therein assert that subsequent experience had induced me to recall my remarks, and that I had lately succeeded in bringing out the striæ of some very difficult tests when in balsam. I will now corroborate this by saying that I am convinced that Prof. Bailey is perfectly correct in his statement with respect to balsam tests which must henceforth be recorded in the list of facts. Thus far we are quite agreed; but as Prof. Bailey's allusions extend beyond this point, self-defence will be my apology for taking some notice of them. Referring to me, Prof. Bailey says, 'the error in his arguments will be sufficiently obvious to any one who will trace the course of a divergent pencil out of the balsam instead of into it, as in Mr. Wenham's experiments, and it will then be seen that large angles of aperture are as useful for balsam-mounted specimens as for others.' Surely Prof. Bailey cannot have well considered this extraordinary, because extremely incorrect, assertion, which is tantamount to saying, that a divergent pencil of rays from a luminous point, submerged in balsam will in each case continue their course in the same straight line without suffering any refraction after emerging from a plane surface of the medium. This is contrary to all reason, for in the trigonometry of optics where there are sufficient data connected with the position and direction of the rays, it comes to precisely the same thing whether they are traced into the refractive medium or out of it. But taking Prof. Bailey on his own statement, I will explain what is the real effect in the case. Suppose a series of rays diverging from a balsam mounted object; from the mean

By request of Prof. Bailey.

refraction of the balsam and glass cover (the indices being about 1-54 and 1:53) total reflection would take place from the upper surface of the latter at an angle of very nearly 41° from the perpendicular. This therefore at once limits the angle of rays collected by the object glass to 82°, aud as total reflection begins where refraction ceases, all rays beyond this point will be entirely reflected down again into the balsam and lost by dispersion; and the extreme rays of the pencil of 82° that just exceed total reflection by passing through the glass, so far from continuing their course in a straight line, are brought down by refrac tion to the very level of the top surface of the cover itself, so that if it were possible to use an objective of 180° of aperture, the effect of the balsam mounting would reduce it at once to 820, and allowing for all possible variation of the refractive power of the balsam and cover, I have no hesitation in affirming that any object mounted in the usual manner in the medium, has never been seen with an angle greater than 85°; but in all probability the extreme limit has been about 78°. This statement is not the result of mere hypothesis, but admits of ocular demonstration, by experiments that will prove it at least half a dozen different ways, and is so true in theory, that to endeavor to disprove it will be to take the difficult course of attempting to undermine the ground upon which I stand, by denying the first laws of refraction upon which my assertion is based.

Prof. Bailey has, no doubt, experienced the advantage of the utmost extent of aperture that can be obtained, in that particular department of investigation in which he has so eminently distinguished himself; and I am willing to admit that if the highest powers are to be used only for viewing thin and flat objects, like the Diatomacea, the aperture may be as near to 180°, as may be practically convenient for this especial purpose; but considering all the requirements and perhaps more useful application of the object glass, I am still of opinion that beyond 150° there is no real advantage to be gained. I have expended much time and taken special delight in the cultivation of the largest apertures, and possess an assortment ranging up to the greatest possible limit, and I can even now bring out striæ with 150° as readily as with anything beyond it, with the positive advantage of a greater distance between the first lens and object. Some of the phenomena described in my communication to the present Journal are extremely severe tests of all the good qualities of an object glass, and yet I have had some whose performance is unrivalled upon a difficult diatomaceous test, repeatedly break down and fail in their effective duty when applied to the investigation of plant-circulation, from the fact of their possessing too much aperture.

ARL. XIV. Remarks on Mr. Wenham's paper on Aperture of Object Glasses; by Professor J. W. BAILEY.

As Mr. Wenham now frankly admits the correctness of my statements with regard to the possibility of resolving difficult test-objects even when balsam-mounted, no further remarks are necessary upon that point, but a few words of comment are required by other portions of his paper.

That my reply was written before I could have had any knowledge that Mr. Wenham had recalled his remarks in which doubt appeared to be thrown on my positive statement of facts will sufficiently appear by the date of my reply, which was published in the American Journal of Science for January, 1855, the very time in which Mr. Wenham's retraction of his remarks appeared in the Quarterly Journal for Microscopic Science.

If Mr. Wenham finds anything objectionable in the form of my reply, he should bear in mind that this discussion is not one of my seeking, and that I put the best possible construction upon his remarks which seemed to call in question the correctness of my assertions. I am utterly averse to anything like scientific controversy, and would make no further remarks in this connection, if Mr. Wenham had not so entirely mistaken my statement, as to represent me as having published sheer nonsense. The statement on which Mr. Wenham animadverts is as follows. "The error in Mr. Wenham's arguments will be sufficiently obvious to any one who will trace the course of a divergent beam out of the balsam, instead of into it, and it will then be seen that large angles of aperture are as useful for balsam-mounted specimens as for others." This statement as it stands, I still hold to, but I must protest against its being considered "tantamount" to any such absurdity as that into which Mr. Wenham has translated it, which is indeed "contrary to all reason."

I mean to assert, however, what Mr. Wenham so emphatically denies, viz., that it does make a difference whether rays are traced into a refractive medium or out of it. I cannot admit that these two cases "come to precisely the same thing."

Mr. Wenham surely does not need to be told that if "the trigonometry of optics" establishes anything, it proves that the same medium which bends an incident ray towards the perpendicular when it enters, will bend it from the perpendicular when it Hence a beam of divergent rays from a point within a medium is rendered still more divergent when it emerges, and in fact is spread out so that the extreme rays which emerge, are in the plane of emergence, or make an angle of 180° with each

other.

SPOOND SERIES, Vol. XXI, No. 61.-Jan, 1856.

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