The Microscope. 323 



limit. Higher magnification by the eye-piece may be useful in 

 testing an objective, and may, it is true, to some persons, be 

 available for long continued work ; but I am making a report of 

 personal experience. The only other thing necessary to say 

 here is that usually the less amplification, the better, after a 

 suitable amount is obtained. Hence neither objective nor eye- 

 piece should be of less focal length than will conveniently serve 

 the purpose required. For a botanical laboratory a ^ inch and 

 a Hh inch dry objectives are the best selection for the common 

 work of students. Occasionally higher powers are needed, 

 sometimes running up to the highest and best procurable. For 

 these exceptional cases provision should be made by having a 

 few such objectives at hand, but students need not be furnished 

 with them as with those first named. Really serviceable magni- 

 fication seems to reach its limit in about a Ath inch or at most 

 an iVth inch objective. Only in rare cases is anj^thing of higher 

 power than a yoth inch, of best quality, eflfectively superseded — 

 with me in nothing but certain studies upon bacteria. 



ANGLE OP APERTURE. 



It appears to me that something similar can be said of the 

 angle of aperture. In the matter of difficult resolution with 

 oblique light, high class and even medium grade objectives have 

 been, in my hands, proportionally successful in just about the 

 order of their aperture, though exceptions have been noted. 

 But for most other uses, it does not appear that the angle of 

 aperture should be relatively rated so high, in the qualities of 

 objectives. It must not be inferred from this that wide angles 

 are, in and of themselves, injurious for biological work. Other 

 things being equal, I should always prefer them, cheerfully put- 

 ting up with any lack of penetration, and, to a certain extent, 

 with inconvenient working distance for the other advantages 

 offered; but crispness of outline, of even the smallest bacteria, 

 depends upon something else quite as much as upon the aperture 

 and cost price of an objective. These smallest bacteria measure 

 about 50000 inch, or about the distance apart of the dots from 

 centre to centre of Pleurosigma angulatum. We all know that 

 great angle is not necessary upon objects of this size. The 

 question is whether excess of angle above a certain essential 

 degree, is of any importance whatever, or indeed, whether an 



