ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 297 



with it ? A perception, that with glass of greater range of refractivo 

 and dispersive indices than any we possessed, we might not only secure 

 great numerical apertures, but secure them dovoid of all colour ; that 

 we could not only annul the primary, but also the secondary and tertiary 

 spectra. It need not surprise us then, that in a country where such 

 splendid theoretical and mathematical work had been done by experts on 

 the principles of microscopic lenses and the laws of their construction 

 and use, even the Government should be convinced that the time to aid 

 the optical expert had come ; that theory had demonstrated the practical 

 possibility of a great improvement in the construction of lenses. The 

 sum of 6000/. was granted by the German Government to Abbe and his 

 collaborateurs, and with, as we have reason to believe, an equivalent 

 outlay on Abbe's own part, the new glass was prepared ; and the new 

 apochromatic lenses with their systems of compensating eye-pieces 

 devised. 



" It is in no spirit of boast, but rather in a spirit of humiliation and 

 regret, that we say that we havo examined many of those apochromatic 

 objectives of all the powers made in Germany, and we have examined 

 all the principal ones that have, since the now glass has reached London, 

 been made there ; and we are bound to say that the English work, based 

 on the principles laid down by Abbe, is so fine as to make the regret 

 immeasurably keener that English micr iscopical literature has been for 

 all these years a blank for practical purposes, on tho theory and 

 principles of optical construction, and on the theory of microscopical 

 observation and interpretation. Such a paper as that of Prof. G. G. 

 Stokes, P. U.S., on the question of a theoretical limit to the apertures of 

 microscopic objectives * from its very loneliness only gives emphasis and 

 point to our contention. Those who have any doubt of the full force of 

 what we are here contending for, have only to compare a dry 1/6-in. 

 objective, say of twenty-five years ago, made by the best makers in 

 London, with a well-chosen water-immersion of ten years ago ; and both 

 these with a recent homogeneous glass of the same power with a 

 numerical aperture of 1*5. Or still better, a dry 1/50-in. objective, of 

 the same date and the same makers, of numerical aperture ■ 98, with a 

 water -immersion lens of the same power of say ten years ago, having an 

 aperture of 1 ■ 04, and a recent homogeneous 1/50 in., with a numerical 

 aperture of 1*38. Still more strikingly, let the same observations be 

 made with a dry 1/12 in. objective of twenty years ago, with a numerical 

 aperture of 0*99, and a homogeneous lens of the same power, with 

 numerical aperture 1*5; and finally, both these with an apochromatic 

 objective of the same power by the same London makers, aud an aperture 

 of 1-40. We venture to say, to histologist, bacteriologist, diatomist, 

 and all other serious workers with the Microscope, that there can be no 

 proper comparison of the results ; or, rather, the comparison is odious 

 indeed for the oldest, and even tho elder lenses. 



"But, as wo have stated, it is to Germany we are indebted for the 

 knowledge out of which, alone, these improvements could havo arisen. 

 In spite of the length and abundance of English treatises on the Micro- 

 scope, it has never been part of the scopo of the respective authors to do 

 other than make tho scantiest reference to the principles of the Micro- 

 scope; and nothing is found that will elucidate the theory of tho 



* See this Journal, 1878, p. 139. 



