A New Form of Small Pocket Sjoeclroscope. By H. C. Sorhy. 65 
arrangement required for opening and shutting the sht, and for 
drawing and pushing one tube inside the other for focal adjustment, 
the total length of the instrument when closed up for putting into 
a case is at least 2\ inches. There is thus length enough for a 
five-prism arrangement, and when that plan is adopted there is 
really no space lost. If such a length is thought to be no incon- 
venience, the ordinary arrangement is on the whole the best ; but 
I would point out the very great advantage of having the lens made 
achromatic, since then all parts of the spectrum can be seen in 
tolerably good focus at the same time. However, when such an 
instrument is inside a brass box, the total length becomes 2 J inches, 
or even more, and f inch in diameter. This becomes somewhat 
inconvenient when constantly carried in the waistcoat pocket. The 
arrangement which I proposed, and which has been carried out for 
me by Mr. Hilger, secures all the advantages of the larger instru- 
ment, and yet gives a length when closed up of less than IJ inch, 
so that when in a small brass box the length is only a trifle more 
than 1\ inch, and the diameter f inch. The actual bulk is thus 
not one-half that of the old form, and is such that the instrument 
can without any inconvenience be carried along with other things 
in the waistcoat pocket, always at hand to examine anything that 
may accidentally present itself. Such small spectroscopes have 
indeed been made before, but the diminished size was obtained by 
sacrificing other advantages, and by making the focal length of the 
lens too small. The plan which I have adopted enables us to 
have the instrument very small, without sacrificing definition or 
brilHancy. This is accomplished by placing the compound direct- 
vision prism between the slit and the lens, so that the entire length 
of the instrument is that of the focal length, and not, as in the old 
arrangement, the length of the prism added to this focal distance. 
Perhaps this method of mounting a prism may appear to be very 
unusual and heterodox, but it does really give a very satisfactory 
result. The prisms consist of a single very dense flint prism of 
106° and two crown of 98°. This combination gives quite as great 
a dispersion as is desirable for the purposes to which such an in- 
strument is likely to be applied. The eye lens is made achro- 
matic, and of If inch focal length, which is increased to 2 inches 
by looking through the prisms, for which it is of course properly 
corrected. It then shows the principal Fraunhofer Hues quite 
distinctly. A, D, F, and G being visible at the same time, but 
the efiect of looking through the prism is to make the focal length 
greater in one direction than in another. In order to bring the two 
sides of the spectrum into focus along with the spectrum itself, it is 
therefore necessary to insert below the prisms a plano-cyhndrical 
lens of 2J-inch focus, with its axis in the line of the axis of the 
prisms. This makes the focus If inch long in every direction ; 
