64 Transactions of the Boyal Microscopical Society. 
II. — On a New Form of Small Pocket Spectroscope. 
By H. C. SoBBY, F.E.S., Pres. E.M.S. 
(Taken as read before the Royal Microscopical Society.) 
In carrying on inquiries witli the spectrum microscope it is often 
very useful to have a small pocket spectroscope, to examine liquids 
in test-tubes or bottles, in circumstances that would make it very 
inconvenient to examine them with the microscope itself. By con- 
stantly carrying such a small spectroscope in my pocket I have also 
often been able to learn valuable facts that would otherwise have 
been overlooked. Medical practitioners might often at once decide 
whether a suspected solution contained blood, and could recognize 
small quantities of it, or of the degraded bile pigments, met with in 
urine in some diseases, by merely looking at it in a test-tube or 
bottle. But since one may carry about such an instrument for a 
long time without anything turning up that need be examined, it 
certainly ought not to be any larger than is compatible with 
efl&ciency. Such a small pocket spectroscope bears much the same 
relation to a complete spectrum microscope as a pocket lens does to 
an ordinary microscope, and therefore a description of a new small 
form of the instrument may, I think, be looked upon as sufficiently 
connected with the subjects treated of at the Koyal Microscopical 
Society. 
When examining Mr. Hilger's new contrivance for measuring 
the positions of any lines seen in a spectrum, as shown by him in 
the Loan Exhibition at South Kensington, he drew my attention to 
some small direct-vision prisms which gave a very considerable dis- 
persion and had a relatively wide field, although their length was 
only about f of an inch. He told me that he thought of making 
use of them for small pocket spectroscopes, and it occurred to me 
that it would be possible to mount such prisms in a somewhat un- 
usual manner, which would have the advantage of making the 
instrument very small and compact. 
The usual arrangement in such pocket spectroscopes is to 
mount the prisms between the eye and the lens used to bring the 
sht to focus. The focal length of this lens should not be less than 
1 J inch, and I find 1| a very convenient focal distance. If a higher 
magnifying power is used, the sht must be made inconveniently 
narrow to get good definition ; the irregularities are greatly 
magnified, and the light too much reduced. A longer focal length 
is unnecessary, and undesirable for many reasons. Now with a 
focal length of If inch and the prism placed in the usual manner, 
the length of the instrument when in focus becomes about 2 j inches ; 
and taking into consideration the space necessary for the excentric 
