34 MICROSCOPIC POWERS 



view of one he placed an opalescent screen 

 which he kept oscillating to serve as a 

 secondary source of radiation and to expand 

 the transmitted wave front, causing it to fill 

 the aperture of the second microscope. This 

 latter instrument gave a further magnification 

 of 100 diameters. The oscillation of the ground 

 glass or opalescent screen removed the imper- 

 fections of excessive magnification and rendered 

 invisible its own grain. A diatom (Pleurosigma 

 angulatum) was exhibited, and showed an 

 amplification of 10,000 diameters. Its mark- 

 ings were clearly defined. 



When we remember that this particular 

 diatom is practically invisible to unassisted 

 eyesight, and that its highest amplification 

 in Dr. Dallinger's edition of Carpenter's 

 Microscope and its Revelations is 4,300 

 diameters, we cannot but realise that a 

 very distinct advance has been made. 



To suggest a second answer to the question 

 propounded, I must refer to diffraction grat- 

 ings or microscopic rulings. 



Lines have been ruled by extremely 



