24: THE MIOBOSCOPE. 



subject of achromatism engaged the attention of some of the 

 most profound mathematicians in England. Sir John Herschel, 

 Professor Airy, Professor Barlow, Mr. Coddington, and others, 

 contributed largely to the theoretical examination of the sub- 

 ject; and though the results of their labors were not immedi- 

 ately applicable to the microscope, they essentially promoted 

 its improvement. 



For some time prior to 1829 the subject had occupied the 

 mind of a gentleman, who, not entirely practical, like the first, 

 nor purely mathematical, like the last-mentioned class of in- 

 quirers, was led to the discovery of certain properties in 

 achromatic combinations which had been before unobserved. 

 These were afterwards experimentally verified; and in the year 

 1829 a paper on the subject, by the discoverer, Mr. Joseph 

 Jackson Lister, was read and published by the Royal Society. 

 The principles and results thus obtained enabled Mr. Lister to 

 form a combination of lenses which transmitted a pencil of 

 fifty degrees, with a large field correct in every part; as this 

 paper was the foundation of the recent improvements in 

 acromatic microscopes, and as its results are indispensable to 

 all who would make or understand the instrument, we shall give 

 the more important parts of it in detail, and in Mr. Lister's 

 own words. 



" I would premise that the plano-concave form for the cor- 

 recting flint lens has in that quality a strong recommendation, 

 particularly as it obviates the danger of error which otherwise 

 exists in centering the two curves, and thereby admits of cor- 

 rect workmanship for a shorter focus. To cement together 

 also the two surfaces of the glass diminishes by very nearly 

 half the loss of light from reflection, which is considerable at 

 the numerous surfaces of a combination. I have thought the 

 clearness of the field and brightness of the picture evidently 

 increased by doing this; it prevents any dewiness or vegetation 

 from forming on the inner surfaces; and I see no disadvantage 

 to be anticipated from it if they are of identical curves, and 

 pressed closely together, and the cementing medium per- 

 manently homogeneous. 



" These two conditions then, that the flint lens shall be plano- 

 concave, and that it shall be joined by some cement to the con- 



