80 VISION WITH THE COMPOUND MICROSCOPE 



Their defects are their inevitable incapacity to open up details in 

 structure that can be disclosed with relative ease by the inclusion 

 into an oil immersion,, and especially an ' apochromatic ' objective of 

 great aperture, of the all-revealing diffraction beams excluded by the 

 dry lens of equivalent power. 



With dry objectives splendid results have been attained both in 

 low and high power work ; but all the latter is being advanced upon 

 by revision with lenses of greater aperture in a striking manner. 

 For twenty years we have been urging our best English microscope 

 makers to enlarge the * angle ' of our objectives, and employing 

 them from a 1-inch to a 5 \ r inch focus. We have seen them 

 advance from dry to water immersion, and from this to oil ; from 

 v^-inch, a TjVinch, and a - 1 -inch of N.A. O95 each, and re- 

 spectively to water immersions of N.A. 1'04 and then to * oil 

 immersions' or homogeneous lenses of N.A. 1'38 for the ^5 -inch 

 and 5 Vinch respectively, and ultimately by a ^L-inch with N.A. 

 of 1'50; and from that we have progressed to the apochromatic 

 objectives with compensating eye-pieces. 



Now the objectives with w T hich the earlier work done by the 

 present editor and his colleague, Dr. Drysdale, was effected to 

 which allusion is made only as being the instance with which we 

 have most practical familiarity are still in our possession ; what 

 was revealed by them fifteen, twelve, or ten years ago we can 

 exactly repeat to-day ; and in the general features of the work in 

 the broad characteristics of the life histories of the saprophytic 

 organisms, minute as they are, revision with objectives of IS". A. 1*50 

 and other lenses of the best English and German makers, reveals no 

 positive error, even in the minutest of the details then discovered and 

 delineated. But the later lenses of great aperture and beautiful 

 corrections have opened up structure absolutely invisible before. 



Thus, for example, a minute oval organism from the jnnnr^ 1 to 

 the 5-oVoth f an i llc h * n l n g diameter was known to possess a 

 distinct nucleus ; the long diameter of this was from the ^th to the 

 r Uth of the diameter of the whole body of the organism. In the ol >s< no- 

 vations of fifteen to twenty-five years since the cyclic changes of the 

 entire organism were clearly visible and constantly observed ; but 

 of the nucleus nothing could be made out save that it appeared to 

 share the changes in self-division and genetic reproduction, initiated 

 by the organism as a whole. But by lenses of N.A. 1'50 and the 

 finest apochromatic objectives of Zeiss, especially a most beautifully 

 -corrected 3 mm. and 2 mm., structure of a remarkable kind has 

 been demonstrated in the nucleus, and it has been shown that the 

 initiation of the great cyclic changes takes place in the nucleus^ and 

 is then shared in by the organism as a w-hole. In short, we have 

 discovered as much concerning the * inaccessible ' nucleus which 

 may be not more than, say, a twelfth of the long diameter of the 

 whole organism by means of lower poivers, but greater apertures, as 

 we were able to find concerning the complete body of the saprophyte 

 with dry objectives. 



But in spite of these facts there is a certain class of even high 

 power work in biology from which the dry lens can never be dis- 



