Critical Microscopy. By E. M. Nelson. 



287 



edge of the flame reflected from the plane-mirror ; it is, whenever 

 possible, far simpler and more satisfactory to work with the lamp 

 direct. 



All this centring operation may be performed in less than 30 

 seconds, and a practised manipulator will not require to use a low- 

 power objective, but will employ for this purpose the objective he 

 is going to use for observation. 



Anyone wishing to become acquainted with the elementary 

 practice of critical illumination must learn by heart the appearance 

 of the figures A, B, C, D, E and F. These figures represent the 

 back lens of a Microscope objective when the illumination is 

 centra], as seen either when the eye-piece is removed or by an 

 inspection with a magnifying lens of the Eamsden disk, which 

 appears as a spot of light just above the eye lens. This Ramsden 

 disk is a diminished image of the back lens of the objective. The 

 higher the power of the eye-piece the greater is this diminution. 

 The following table explains the figures : — 



Example. — P.angulatum is placed underneath an apochromatic 

 ^ in. of X. A. • 65. With a cone as at D a false resolution of the 

 diatom is seen, the image being lines crossing one another at an 

 augle of 60°. When the W.A. is increased to H a true image, so 

 far as it goes, is obtained. It consists of isolated black dots, each 

 hole in the network being represented by a black clot.* The ^ in. 

 must be a good lens to give a sharp picture of this view of the 

 object under these conditions. 



A novice will probably call the figures A, B and H hair-splitting, 

 but if he tries them he w r ill find a much greater difference in the 

 quality of the image than he imagined. Now, G shows an objective 

 divided into equal areas, the area of the annulus being precisely 

 equal to that of the central disk, therefore half the illuminating 

 power of the whole objective resides in that narrow black annulus ; 

 then as the black annulus in A represents less than one quarter 

 of the illuminating power of the whole objective, it follows that 



* Tbe following measurements very carefully performed witb a long-tube oil- 

 immersion j'-j in. may be of interest. Transversely tbe rows of dots count 45,260 

 per in., diagonally in botb directions 45,720 per in. Tbe mean of tbe tbree direc- 

 tions being 45,490 per in. or 1791' 5 in mm. 



