160 MICROSCOPIC VISION. 



depends on its capability of standing a large cone of illu- 

 mination, and the difference between a really fine objective 

 and a mediocre one is that the fine objective will stand at 

 least I of its back lens being filled with light, while the me- 

 diocre one will not ; so then if wide cones of illumination are 

 injurious, and only narrow cones are to be used, it follows 

 that mediocre objectives are quite as good as those of the 

 first class, because with a narrow cone it is impossible to 

 tell the difference between them. Important as this is from 

 a brass and glass point of view, it sinks into the shade in 

 comparison to the manufacture of false images by use of a 

 narrow cone. 



As there is a diversity of nomenclature in use in matters 

 appertaining to these discussions, it will be as well first to 

 clear the ground of all ambiguity. The central white beam, 

 sometimes called the " central maximum," will be called the 

 dioptric beam (Z>) ; the first coloured spectrum next the 

 dioptric beam will be called the spectrum of the 1st order 

 (1) ; the one next to that will be called a spectrum of the 

 2nd order (2), and so on ; see Fig. 2. 



The law, then, for the manufacture of the simplest form of 

 false image, is the union of a spectrum of the 2nd order with 

 the dioptric beam, when that of the 1st order is suppressed. 

 In this case the false image will consist of a doubling of line 

 structures, and the insertion of an intercostal in hexagonal 

 and similar structures. To repeat, — the cause of the false 

 image is the suppression of the 1st order spectrum, and not 

 merely the admission, per sc, of that of the 2nd order ; for 

 if the 2nd is admitted and combined with the dioptric beam, 

 the 1st also being combined with them, there will be no 

 false image. The suppression of spectra of the first order 

 may be easily accomplished by placing a suitable stop at the 

 back of the objective ; there is, however, another and far 



