I should like to know if Mr. Green finds a 6 diameter reflector equal to 1 course different eye-pieces may give diff THE PHENOMENAL Mr. C. L. Prince's remarks, taken ov Register for August, on the probable cau glow, besides giving a new explanation to are full of interest. Yet, when we con moderate estimates, the total weight, or will be required to be distributed, in th dust, or crystals, over the whole atm earth, and in sufficient quantity to prod or refraction (as Mr. Prince asserts), it suppose that the Krakatoa outburst cou propelling force, within so short a time matter, saline, or whatever else, to be s enormous masses get distributed all ov being precipitated to the surface, on acc of the whole mass, or volume ? Be the cause what it may, we must mystery sooner or later. There seems of evidence, which render Mr. Prince's since February, I have seen, and so hav dependent observers, both here, in Eu evening soon after sunset, a bow of This bow or arc being of a dull brick-re that whatever the reflecting medium m opaque matter. The glow surrounding the sun, as seen in daytime, seems to point to a similar conclusion. Again, Mr. Prince says, that during the winter months of the northern hemisphere the glow was more intense, probably because the cold weather caused a greater amount of crystallization of the saline substances. That this may be probable, I will not dispute; but the reverse has been the case in the southern hemisphere. During the summer months here, the glow was at its greatest brilliancy, while as winter advanced it dulled considerably. The variable character it has displayed all through must also be considered. It has occurred to me, that if we suppose the reflecting media to float in huge semi-detached cloud-like masses, this variability will be explained. If so, we shall be able to study the direction and motions of the upper strata supporting the dust. Rolontein, Transvaal: Sept. 26, 1884. REVIEWS. JOHN BALLOT. Experimental determination of wave-lengths in the invisible prismatic spectrum. By S. L. Langley. (From the American Journal of Science, vol. xxvii., March, 1884). pp. 20. The prism employed by Professor Langley, was made by Adam Hilger, of London, and its optical properties are in every way satisfactory. It is of a white flint, which has proved singularly transparent to the longest solar waves. The professor describes an apparatus devised by himself for the measurement of obscure wave-lengths. He could thus test the accuracy of the various formulæ connecting refraction with wave-length, and he finds the formula of Canchy gives grossly erroneous results when extended far behind the limits within which the observations on which it is founded are made. That of Redtenbacher, while satisfying the observations in the visible spectrum, fails when extended to the invisible, and is even less satisfactory than that of Canchy. Briot's formula, though not exact, yet gives results much more trustworthy than the others. Plate V. presents the curves represented by the different formulæ, showing their actual errors by inspection, and that in reality we can scarcely assign any limit to the extent of the infra-red spectrum, and that far from the curve having an asymptote parallel to the axis of X as Canchy's theory requires, one curve (so far as we can follow it) rather tends to ultimately coincide with a straight line cutting the axis at a finite angle, and (if this axis pass through the point n = 1) at a great distance from the origin. [n is the index of refraction corresponding to the deviation.] Every prism gives a different map of the spectrum, and the general conclusions here offered, as to the relation of wave-lengths and indices of refraction, have been drawn from the observations on a single prism, and have not been experimentally verified on others. This is on account of the extremely slow and laborious character of the process used (which has involved some months of labour for this special prism). Though there seems no reason to doubt the generality of the conclusions, it may be hoped that these experiments will be repeated with prisms of other |