140 



TESTING THE MICROSCOPE. 



B. XINETEEX-BAND PLATE. 



It is evident from these tables that the test-plates of Nobert are 

 not inferior in fineness of division to the most difficult organic 

 objects. They agree with them, also, in that the image of the 

 lines is a pure interference image, produced by the diffracted rays. 

 We must, however, leave it undecided, not being ourselves intimate 

 with the subject from personal observation, whether they really 

 offer a more reliable standard than the siliceous envelopes of 

 diatoms. According to Max Schultze 1 the equality of the later 

 plates with nineteen bands leaves very little to be desired. That 

 a complete uniformity of the corresponding bands of lines, 

 especially with regard to the strength of the lines, cannot be 

 attained with any mechanism, may, cl priori, be asserted with 

 certainty. 



2. RELATIVE POWER OF DISCRIMINATION. 



Although in the foregoing the present standpoint of practical 



Optics is to a certain extent arithmetically defined by the results 



obtained with images of wire-gauze, in so far as the limit of the 



performances of high-class Microscopes can be determined in 



1 " Archiv fur mikr. Anat." Bd. i. pp. 3-4. 



