74. SELECTION AND USE 



attention called to this subject, and after careful study lie con- 

 cluded that if he could only procure a durable glass of greater 

 refracting power than that ordinarily attainable, the angular 

 aperture might be greatly increased. He at once went to work, 

 and after many experiments he succeeded in producing a glass 

 which enabled him to attain immediately an angular aperture 

 of 146. As early as 1857 he had produced a l-12th with an 

 angular aperture of 178. His objectives had corresponding ex- 

 cellence in other directions, and from that time forward this 

 country has been noted for the excellence of its objectives, and 

 especially for their great resolving power. We may note, in 

 passing, that glass of great refractive power, combined with 

 sufficient hardness and durability, is now produced as a regular 

 article of commerce. 



Penetrating Power. As previously stated, penetrating 

 power, in the early days of microscopy, meant precisely what is 

 now understood by resolving power. Now, however, penetra- 

 ting power is usually understood to mean the extent to which 

 an object-glass shows the depth or thickness of an object. It 

 is obvious that such a result can only be produced by the lens 

 showing several layers of images, all of which are equally in 

 focus, and consequently equally visible at the same time. In 

 other words, instead of bringing only one gifen point sharply 

 to a focus, the lens, which has great penetrating power, will 

 bring several other points lying above and below this point 

 equally to a focus. Hence, as Dr. Pelletan well observes in his 

 work, " Le Microscope ": " From a purely theoretical point 

 of view, an objective with penetration is in reality a defective 

 objective. " 



As this subject is one of great importance, we have intro- 

 duced the two engravings, Figures 18 and 19, for the purpose 

 of making it clear to our readers. In Figure 18 we have shown 

 a small lens of half-inch focus, and with a very narrow angle. 

 Figure 19 shows a lens of the same focal length, but with an 

 angle of 90. The points at Avhich the lines a c and b c in Fig. 

 18, and of d and V cf in Fig. 19 cross each other, are the respec- 

 tive foci of the two lenses at the centre of the field of view. 

 Across the points where the lines a c and b c, and a' c' and b' d 



