﻿Improved Form of Lit trow Spectroscope. 139 



characteristics of the original Littrow form, haveheen entirely 

 lost in the attempt to overcome the difficulties already men- 

 tioned in regard to the illumination of the field and the 

 inconvenient position of the eyepiece. 



More recently Prof. Bracket* has, without essentially 

 altering the original design of the instrument, overcome the 

 first and principal difficulty, by making the combined colli- 

 mator and observing lens with faces of such curvature as to 

 reduce the quantity of light reflected to the observing eye- 

 piece to a minimum. 



It recently occurred to me, in connexion with an attempt 

 to photograph the spectrum with a spectroscope of this form, 

 that the difficulty could be completely overcome by the use 

 of a concave mirror in place of the lens, and a trial of an 

 instrument so constructed showed that it possessed other 

 advantages also. 



The following brief description of this form of instrument, 

 which is, I believe, new, may therefore be of interest. Fig. 2 

 (PJ. VI.) is a plan view, the top plate of the containing box 

 being removed to show the parts clearly, and iig. 1 is a side 

 view. A small right-angled prism a receives the light from 

 the slit s, and reflects it to a concave mirror b which acts as a 

 collimator. From b the collimated beam is reflected to the 

 prism c, behind which is the plane mirror d, which returns 

 the rays again through the prism to the concave mirror, by 

 which they are finally brought to a focus at the observing 

 eyepiece /, which is placed just above the plane of the slit 

 and near the edge of the prism. A blackened screen L is 

 placed between the prism and the eyepiece to cut off the light 

 reflected from the faces of the former. A second screen, L, 

 is placed between the eye and the slit, or else the beam of 

 light is brought to the latter through a tube or box, as indi- 

 cated by the dotted lines. Diaphragms m, n are also placed 

 in front of the eyepiece to cut off any general light diffused 

 from the walls of the enclosing box, which are of course 

 carefully blackened. 



It will be readily seen that with this arrangement of appa- 

 ratus, the only light which can reach the eyepiece besides that 

 which forms the spectral image is that which is diffused by 

 the reflecting surface of the mirror. If this mirror is pro- 

 perly silvered f the general illumination due to this cause 

 will be insignificant, and the field of view will be quite as 



* Loc. cit. 



t In order to secure a non-diffusive coat of silver, it is necessary to so 

 manage the silvering bath that the deposited film is bright as it comes 

 from the solution and requires no subsequent polishing j for no matter 



