114 PRACTICAL MICROSCOPY. 



brought into focus simultaneously. Until very recently 

 the property of penetration was shrouded in mystery and 

 its usefulness often exaggerated, but thanks to Professor 

 Abbe, who has made it a special study, most of the diffi- 

 culties surrounding the subject have now been cleared 

 away. 



The greatest use of "penetration" is perhaps in the 

 employment of the binocular microscope, for it is only 

 when an object can be seen in its entirety that a true 

 stereoscopic image can be obtained. There is no doubt 

 also that this penetrating power is very useful for general 

 work, such as dissecting; still it should be remembered 

 that a low - angle lens with much penetration will not 

 usually stand deep eye-pieces. Dr. Blackham writes of 

 "depth of focus" as "only a residual error at best" and 

 "that it can be left in a wide-angle lens as well as in a 

 narrow-angled one." Whether this is practically correct, 

 perhaps opticians will be able to tell us, but it seems to the 

 author much more difficult to leave spherical aberrations 

 uncorrected and practically unnoticed in a wide-angle lens 

 than in one of narrow angle. 



Professor Abbe has demonstrated the fact that penetra- 

 tion or " depth of vision " depends upon two circumstances : 

 first, the accommodation depth of the eye, and secondly the 

 depth of focus, varying with the quality of the images pro- 

 duced from certain distances on each side of the exact 

 focal plane. The eye, being insensible to small defects in 

 the various images, can practically discern the object in its 

 several depths, provided the images compare with sufficient 

 exactness. 



The accommodation depth is very great with low ampli- 

 fications. Under a magnifying power of 10 diameters it 

 amounts to nearly 2 I millimetres, while with higher 

 powers the image passes quickly into a mere transverse 



