THE MICROSCOPE. 23 



their comparatively small cost and good quality. The 

 lenses made in England, especially those by Ross, R. and 

 J. Beck, Powell and Lealand, are of great excellence : Wen- 

 ham's -f made by Ross, the new ^ by Powell and Lealand, 

 and the ^ by R. and J. Beck, are all lenses of such 

 beauty as has, in the opinion of competent authorities, not 

 been equalled elsewhere. 



37. Angle of Aperture. The angle of aperture of 

 an objective is the angle formed between the most external 

 rays that can penetrate its entire system of lenses from a 

 luminous point placed in the focus; thus, in Fig. 26, rays 

 are seen diverging from a luminous point 

 in the focus/: some of them (a a'} enter the 

 first or front lens, too near its periphery to get 

 through into the second. The rays b b' 

 are the most peripheral rays that are capable 

 of passing through all the lenses of the ob- 

 jective, and therefore the angle between b 

 and b' is the angle of aperture : from this it 

 appears that the actual aperture of a lens is 

 a very different thing from its angular aper- 

 ture. There has been much discussion re- 

 garding the relative merits of lenses of small 

 and of large angular aperture. With a large 

 angle more of the oblique rays are admitted, 



\ . P c , 7,./j/---\r FIG. 26. Angle of 



and therefore for the resolution (definition) of aperture of the 

 very fine lines, such as those on diatom $f ivc ' 

 scales, and the rods and cones of the retina, 

 the larger angle is preferable ; but the more the angle is 

 increased, the greater is the loss of penetrating power ; by 

 penetrating power is meant the power of seeing with toler- 

 able distinctness points lying a little above and below the 

 exact focus. This is a matter of very great importance in 

 studying the structure of tissues and organs. It may be 

 readily understood from Fig. 26 that the smaller the angle, 

 the more can the objective be raised or lowered without 

 throwing the object much out of focus. At the same time, 

 however, lenses of large angle give a more brilliantly illu- 

 minated field, and are on that account more agreeable to 



