﻿338 Mr. F. L. 0. Wadsworth on 



may easily bring different parts of the spectral field to the 

 cross wires of the observing telescope by a rotation of the 

 prism alone ; bnt in none of the usual forms without violating 

 the usual condition imposed in spectroscopic work, viz., that 

 the prism shall always be in minimum deviation for the 

 central ray in the field. 



Heretofore the only forms of " fixed-arm " spectroscopes 

 which fulfil the latter condition have been of the Littrow 

 type, in which the ray is. by a system of reflecting surfaces, 

 made to traverse the prism twice in opposite directions, and 

 to emerge finally in a direction parallel but reversed in 

 direction to that in which it entered. 



The original form and the various modifications of this 

 instrument have been already described *. While they 

 fulfil the condition of bringing any part of the spectral 

 field into the field of the observing telescope without changing 

 the angle between the latter and the collimator, there are 

 certain objections to all of them. The most serious objection 

 to the original form, that of a general illumination of the 

 field, has been simply and successfully overcome in the 

 manner already pointed out by the author "f, but the second 

 objection, that of a close proximity of the slit to the observing 

 eyepiece, remains. Young's and Browning's modifications 

 overcome this latter as well as the former difficulty; but 

 they are both complicated and expensive, necessitating as 

 they do the use of at least three reflecting-surfaces and 

 two independent telescopes, in addition to the usual prisms. 

 Moreover, with all instruments of this general type, it is 

 necessary for the beam of light to pass twice through the 

 prism or train of prisms, and this, where brightness is of more 

 importance than large dispersion, as with very faint sources 

 of light for example, is a disadvantage. For these reasons 

 I have thought it would be of interest to describe some 

 simpler forms of " fixed-arm " spectroscopes, which I have 

 recently devised and used, and in which the usual arrange- 

 ment of parts may be preserved, viz., a separate collimating 

 and observing telescope, on opposite sides of a prism-train, 

 through which train the light is passed but once. The slit and 

 observing eye-piece are thus far removed from each other, 

 and the axes of the observing and collimating telescopes 

 may be arranged to make any fixed angle with each other. 



The general solution of the problem is effected by the 

 introduction into some part of the spectroscopic train, between 



* "An Improved Form of Littrow Spectroscope," F. L. 0. Wadsworth, 

 Phil. Mag. July 1894. 

 + Ibid. 



