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instrument could be governed at the same time that the image 

 was closely inspected. The first of these objects is secured by giving 

 a very large angular aperture to the system of lenses used for col- 

 lecting the light, and by using another set of lenses for condensing 

 it again on the object, and so arranging their focal length in pro- 

 portion to the focal length of the object-glass, as to cover the plate 

 to be acted upon, and that space only : the second, by using a sort 

 of eye-piece for enlarging the image formed by the object-glass. 



The lenses divide themselves into four groups, as represented in 

 the figures, in which the light is supposed to proceed from the left 

 hand to the right. The first set for collecting the light is composed 

 of three large lenses, a meniscus, plano-convex and double convex, 

 being a combination of three lenses similar in effect to Herschel's 

 doublet ; the second set for condensing the light on the object is a 



similar set of lenses, but of much shorter focal length, and turned 

 the other way ; between these two sets is a plano-convex or plano- 

 concave lens placed at its focal length from the convergence of the 

 rays from the first group, so as to make the rays pass to the con- 

 densers in a state of parallelism, and so do away with the necessity 

 of changing the distance between the collectors and condensers 

 for each adjustment of the latter : the third group forms the object- 

 glass, which must be so corrected as to have the rays of the spec- 

 trum between the fixed lines G and H as much as possible brought 

 to a point, as these rays are those that produce the maximum ac- 

 tion on the silver salts used in photography ; this will require the 

 red rays to be left untouched, just in the same way as Fraunhofer 

 left those of the blue end of the spectrum dispersed in correcting an 

 object-glass for light. The fourth group is the common eye-piece 

 left under-corrected. A rather better form for this is a Ramsden's 

 eye-piece with the first lens partially achromatized, by making it a 

 compound lens with the radius of curvature of the common surface 

 nearly double that of the surface that would render it achromatic. 



