32 



THE PRESIDENT S ADDRESS. 



balsam with a 3/4 cone from a Powell fluorite apochromatic 

 condenser (1/4 of 0'95 N.A.) : — 



Objective. 



Apochromatic lin. ... 



1. Achromatic 4/ 10 (1875) 



2. Apochromatic 12 



3. Semi-apochromatic 1 4 

 Achromatic 1/4 (1875) 

 Semi-apochromatic 17 



4. Achromatic 1/5 

 Apochromatic 1/4 



5. Semi-apochromatic 1/12 



6. Apochromatic 1/8 



White Light. | Blue Light. 



22,000 



40,000 strong 



46,000 



53,500 



53,000 barely 



60,000 



60,000 



65,000 



90,000 barely 



94,000 



25,000 

 49,000 

 53,500 



60,000 barely 

 60,000 barely;: 

 65,000 

 65,000 barely 



1. Would resolve probably 42,000 with white light (construction same as 

 achromatic 1/4, viz., triple front and back, double middle). 



2. A very fine lens. 



3. A little more than 3/4 cone used ; this lens is a very strong resolver, 

 and stands blue light even better than some apochromatics. 



4. A fine example of an achromatic by Gundlach. 



5. Will not resolve the Nitzsehia curvula, 90,000. 



6. Resolves Amphipleura pellucida, 93,000-95,000. Less than 3/4 cone 

 used. 



To return to the diffraction theory, it has been recognized for 

 some time past that there is more than one kind of image, and 

 on a former occasion it was my endeavour to prove to yon that 

 there were three distinct kinds of images, one being a " true " 

 image, which went in and out of focus as a daisy under a 4in., 

 the other two being " false " images, one of these a " true-false " 

 image, whose character was similar as regards the arrangement 

 of the elements of the periodic structure to that of the object 

 itself, but under focal alteration it passed into another kind, 

 called a "false-false" image, whose character entirely differed 

 from that of the " true " image. Now Mr. Wright's position, 

 if I have interpreted him correctly, is this, that his new " true " 

 image comes under the laws of the Airy or physical theory, but 

 the other two images, viz., the "true-false" and the "false- 

 false," conform to those of the Abbe or diffraction theory. 

 With regard to this last image, it is admitted by all genuine 

 microscopical workers that it is not only of no use, but is abso- 

 lutely a hindrance to the interpretation of microscopical struc- 



