﻿138 Mr. F. L. 0. Wadsworth on an 



reflector, and after reflexion retraverse the train in the oppo- 

 site direction, and are brought to a focus by the same lens 

 which acts as a collimator. To observe conveniently an 

 image of the spectrum, a small right-angled prism covering 

 one half the field is placed near the slit. 



In certain respects the advantages of this form over the 

 ordinary one are unquestionably great ; for not only does it 

 allow all parts of the instrument, save the prisms and plane 

 reflector, to be fixed in position, but it also involves the use 

 of only one lens, a consideration of some importance in the 

 case of large instruments. Unfortunately in this, its original 

 form, it has two very practical disadvantages : — 1st, the 

 general illumination of the field of view by reflexion of a 

 portion of the light from the slit from the surface of the lens ; 

 and, 2nd, the close proximity of the observing eyepiece to the 

 slit. Both of these difficulties are avoided in the modified 

 forms already referred to, in which the collimator covers only 

 the lower half of the prism, the plane reflector being replaced 

 by a right-angled prism, which is so placed as to return the 

 ray after two internal reflexions through the upper half of 

 the prism. The observing telescope is consequently entirely 

 above and distinct from the collimator, and can be placed at 

 any angle with the latter (usually it is placed at right angles) 

 by the use of another reflector. 



This arrangement, while entirely avoiding both of the 

 difficulties mentioned, introduces others of a different cha- 

 racter. In the first place the cost is considerably increased ; 

 for two objectives are required, one for the collimator, the 

 other for the observing telescope ; the prisms for a given 

 aperture of telescope must be twice as large as before ; and, 

 finally, two right-angled prisms both of large size, one having 

 the same aperture as the telescope, the other an aperture 

 about 1^ times greater, are required. This makes the instru- 

 ment more expensive even than one of the ordinary form of 

 the same dispersion. 



Again, the increase in the number of optical surfaces 

 involved diminishes very considerably both the brightness 

 and the distinctness of the spectral image, and increases the 

 number of adjustments required for accurate spectrometric 

 work. 



For all these reasons it is not surprising that these instru- 

 ments have failed to displace to any extent the standard form; 

 for the only advantage which they possess over that form is 

 their somewhat greater compactness and rigidity, both tele- 

 scopes being fixed in position. But the simplicity of design 

 and cheapness of construction, which were distinguishing 



