648 } Transactions of the Society. 



perfectly, irrespective of other similar objects which may be con- 

 tained in the field of view, and which can, if necessary, be easily 

 focused and scrutinised in turn. 



It is obvious that the effective employment of the beautiful 

 objectives now obtainable is of vital importance to all those 

 engaged in original microscopical research if their results are to be 

 of permanent value, and it is, therefore, hoped that the present 

 communication may prove of interest to inexperienced workers 

 who do not necessarily concern themselves with diatomic structure. 



We all know that with an objective of N.A. 1'4, the practical 

 visual resolving limit for lines ruled on glass does not exceed about 

 120,000 lines to the inch when a screen and nearly full solid illu- 

 minating cone are employed, Grayson's band containing this number 

 of lines having been actually thus seen faintly but completely 

 resolved under a 3 mm. apochromat of N.A. 1"42. Now because 

 a power of about 1000 diameters may be sufficient to just show 

 such lines when it is already known that they are there, and one 

 is sure that the resolution is complete, it has been maintained that 

 anything over a X 12 eye-piece used in conjunction with a |^-in. 

 objective is merely useless empty magnification. Against this 

 theoretical idea we must place the practice of the old leading 

 microscopists, who effected most of their original discoveries by 

 means of a much higher ratio of magnifying power to aperture. 

 The theorist, too, is so accustomed to base all his opinions on the 

 behaviour of ruled gratings, the true nature of which is exactly 

 known to him, that he is apt to overlook the fact that the minute 

 translucent organisms whose structure the biologist is concerned 

 to ascertain, are not built up of a series of parallel lines or regu- 

 larly distributed points, it being one thing to differentiate the 

 several unknown components of such an object, and quite another 

 to just perceive something already known to exist. Thus it has 

 been most truly affirmed that when once a discovery has been 

 made with a high power, a much lower one will usually be found 

 sufficient to demonstrate it. 



The general use of high magnifications may not be particularly 

 popular or desirable from the professional optician's point of view, 

 for imperfections in objectives which in ordinary circumstances 

 may not be obtrusive, and thus invisible to the tyro, are rendered 

 conspicuous so soon as the images are greatly enlarged by the 

 ocular, especially as a very large illuminating cone will then be 

 found necessary in order to secure comfort in observation and to 

 avoid " entoptic " effects. This, together with the great magnifica- 

 tion, strains the capacity of the lens to the utmost, and as a matter 

 of fact many high priced objectives fail utterly under the trial. 

 On the other hand many cheap semi-apochromats perform remark- 

 ably well under these hard conditions, and some few accidentally/ 

 perfect combinations (apochromatic, semi-apochromatic, and even 

 achromatic) will be found to withstand the ordeal in a surprising. 



