IMAGES OF WIRE-GAUZE AS TEST-OBJECTS. 



131 



From this table we arrive at the conclusion, in the first place, 

 that the optical power of the Microscope does not keep pace 

 either with the power of the objectives or with the total magni- 

 fying power. Selecting any particular case, we observe that more 

 than twice the amplification is required to reduce to one-half the 

 diameter of the meshes still just distinguishable ; and conversely, 

 an amplification less than half as great to render recognizable 

 meshes twice as large. We may, therefore, with justice require 

 that the lower objectives shall develope the greatest optical power 

 relatively, and on the other hand not require too much of the 

 higher and very highest ones. If, with the latter, the increase of 

 the magnifying power up to the limit of the attainable, produces 

 a further decided gain, the optician has accomplished all that can 

 at present be expected of him. Moreover, it is seen that the power 

 of discrimination in the Microscope is regulated more according to 

 the focal length of the objective than to the total magnification. 

 The slight increase of distinctness, which is observed on the appli- 

 cation of deeper eye-pieces, is in no proportion to the increase in 

 the size of the image effected by it. 



If, in the second place, we select from the figures those which 

 relate to the highest objectives, with which the smaller instru- 

 ments are usually provided (for instance, Hartnack's No. 7, 

 Beneche's No. 9, Zeiss's D, Gundlach's V, &c.), their focal lengths 

 in nearly every case vary between 3 and 4 mm., and the magni- 

 fying powers between 250 and 350. We may require of such 

 combinations that they shall resolve clearly the meshes of the 



