Recent Microscopic Literature. 329 



affirms, cannot be pushed to extremes without sacrifice of 

 penetration. We believe Mr. Lister has worked out this sub- 

 ject more completely than any one else, and we think we are 

 right in saying that, omitting mere surface markings of the 

 most troublesome diatoms, rales could be laid down showing 

 the most advantageous proportions in which angular aperture 

 and focal distance should stand to each other to ensure the 

 greatest accuracy of definition. This subject is of great prac- 

 tical importance, and we regret that Mr. Brooke's anniversary 

 address was not more explicit in dealing with it. 



The best mode of obtaining great amplification depends in 

 no small degree upon the angle of aperture question. Sup- 

 pose, for example, a power of 1500 or 3000 linear is required 

 for the exhibition of minute structure. How is it best obtained ? 



. Brooke gives a preference to lengthening the body of the 

 instrument, over the employment of very deep eye-pieces ; but 

 upon the subject of deep objectives his statements do not 

 coincide. In his notes on the microscopes of the Exhibition, 

 he tells us that C( no objective yet manufactured for sale at all 

 rivals in its power of development the -^th of Messrs. Powell 

 and Lealand,'" and in the presidential address we find the con- 

 tradictory assertion that he "has not hitherto succeeded in 

 developing any point of organic structure with Powell's ^th 

 that is not equally visible with -j^th by Ross." If j- 2 th of Eoss 

 and -4th of Poweli and Lealand were selected as of equal merit 

 in workmanship, it would still be found that they differed con- 

 siderably in the proportion which their angles of aperture bore 

 to their focal lengths ; and it is difficult to believe that the two 

 proportions are equally advantageous. Messrs. Powell and 

 Lealand' s exquisite ~th is much more limited in its range of 

 utility than their fjh, because the latter will work through 

 thick covering glass, while the former requires it to be so 

 extremely thin as scarcely to bear a touch. Messrs. Smith 

 and Beck's ~t}i, which has a moderate angle of aperture, is as 

 generally applicable as a -^th or a ~fch ; and this constitutes no 

 small proportion of its merit. Mr. Ross's stated in his 



catalogue, has an angular aperture of 1 70 3 . Working angles of 

 aperture are nearly always much less than those calculated by 

 opticians; but suppose Mr. Ross made a -55th of the s: 

 working angle, or less than that of his ^th, it does not 1 

 possible that when used to obtain the same amplification, I 



»uld both be equally advantageous in point of penetration. 

 We have tried and admired Ross's ~th, and that of Powell and 

 Lealand; but when it is desired to see the interior structure 

 and movements of small objects, such as desmids or infusoria, 

 it cannot be a matter of indifference whether a given magni- 

 fication is obtained by a deep objective of moderate angle, 



