﻿142 Improved Form of Littrow Spectroscope. 



With this new mirror the advantages of this form of in- 

 strument were still more apparent, and the results obtained 

 leave little to be desired. With it photographs of different 

 portions of the spectrum, extending from the violet to the 

 red, have been obtained, which show an amount of sharpness 

 and detail which is, I think, considerably greater than has 

 heretofore been obtained with a single prism of the material 

 and aperture of the one here used. 



Two other modifications of the form which have suggested 

 themselves during the course of these experiments are shown 

 in figs. 3 & 4. 



In the first of these the collimator is placed in the position 

 occupied by the plane mirror, and the rays pass directly from 

 the slit through the prism without collimation. The slit and 

 spectral image are therefore situated at the two principal 

 conjugate foci of the mirror. In this form, which has been 

 given only a preliminary trial, the definition is, as might have 

 been expected, decidedly inferior to that in the form of the 

 instrument just described. The advantage which this form 

 offers is simply its great simplicity and cheapness, the number 

 of optical surfaces involved being only 4, In the second 

 form (which is not properly a modification of the Littrow 

 form at all, as the rays traverse each prism only once) separate 

 concave mirrors or lenses are used for the collimator and for 

 the view-telescope. The arrangement is shown in fig. 4. A 

 fixed collimating telescope with slit at s and collimator at a 

 sends the beam through a prism to a plane mirror m, by 

 which it is reflected to a second prism p placed by the side 

 of the first ; after passage through which it falls upon the 

 objective of the view-telescope T, which is also fixed in posi- 

 tion, at an angle with the first equal to the angle between 

 the incident and reflected rays on the mirror m. The prism- 

 table on which the two prisms are mounted is connected with 

 the arm which carries the mirror m by a minimum-deviation 

 attachment, as in the previous forms. A little consideration 

 will show that the central ray in the field of the T will always 

 remain at minimum deviation as the arm carrying the mirror 

 m is revolved. This form has not been actually tried, but 

 would seem to offer certain advantages when it is desirable 

 for any reason to use separate telescopes for collimating and 

 observing. 



Astro-Physical Laboratory, 

 Smithsonian Institution,Washington, D.C. 

 March 1894. 



