Eye-pieces for the Microscope. By E. M. Nelson 531 



Of late it has been the fashion to belittle Gauss's theorem, and 

 to dub it as " merely an approximation," but the above work shows 

 that it is a very close approximation. It is altogether a magnifi- 

 cent working theorem, and one of the best ever devised. 



Since the above was written, a figure of a non-achromatic 

 Microscope has been found in Coddington, Part II. (1830) pi. xui., 

 fig. 190, which has its objective image, as influenced by the field- 

 lens of a Huyghenian eye-piece, drawn with its curvature in the 

 proper direction. 



Unless the text describing this instrument is read, one is very 

 apt to be misled by the figure, for, at first sight, it looks as if it 

 were the representation of a Microscope of the Benj. Martin type, 

 that has a biconvex lens to act as a back lens to front lenses of 

 various powers, the eye-piece being of the Hamsden form, and the 

 tube-length only 1^ in. The text, however, shows that this is not 

 so. The eye-piece is a Huyghenian, with a double eye-lens, so 

 that the lens which was thought to be the back lens of the 

 objective is in reality the field-lens of the Huyghenian eye-piece. 



This Coddington Microscope was made hy Gary, who, for the 

 sake of cheapness, disregarded Coddington's formulae for the forms 

 of the lenses, and made them all equi-convex. Nevertheless, the 

 instrument performs better than any old non-achromatic Microscope 

 I have tried. It is interesting to know that it was the last non- 

 achromatic IVIicroscope ; an example is in the Society's cabinet. I 

 have not been able to trace the author who first made the mistake 

 about the curvature of the image in the Huyghenian eye-piece. 

 It is found in a paper by Cornelius Varley in the Transactions 

 of the Society of Arts, li. p. 189 (1838). In this paper Varley 

 advocates the over-correction of the eye-piece, by making the field 

 lens of flint-glass ; and he also suggests a push-tube adjustment 

 for the distance between the lenses, a device which has lately been 

 reintroduced. The examples I have seen would probably destroy 

 the image given by any Microscope objective. 



Oct. 16th, 1907 -2 N 



