490 DUNCAX J. REID ON CRITICAL ILLUMINATION IN 



light. This I find very irritating, and much prefer the uniform 

 illumination of the collecting-lens image. 



As a collecting lens one may use an ordinary bull's-eye ; but 

 what is better is one like the Nelson lens, which gives much less- 

 spherical aberration. A lens I have found to give very good 

 results as a collecting lens is Watson's macro-illuminator, which 

 is a low-power achromatic substage condenser of 2-inch focus. 

 The Zeiss collecting lens, on stand, is one which is rather exten- 

 sively used ; but it is of a much longer focus (4 inches) than the 

 Nelson lens, and therefore requires that it and the lamp should 

 be farther away from the microscope, to produce the same size of 

 image of flame, than is necessary with the Nelson lens, w^hich is 

 only of 1 and l/4th inch focal length. 



It is better to have on the collecting lens an iris diaphragm, 

 which facilitates the final adjustments. 



Messrs. Zeiss have lately introduced a new form of Nernst 

 lamp, with an aplanatic collecting lens with a shorter focal 

 length, as far as I know, than any other on the market, which 

 should be very useful where space is limited. It gives a very 

 sharp image of the Nernst filament on the back of the substage 

 condenser. 



3. The Principles of Correct Illumination. 



A. Of tlie Field. 



As I have said, I prefer the field to be fully and uniformly 

 illuminated, and it is quite possible to do this for all powers with 

 the edge of the flame, even when critical illumination is employed, 

 by the use of a collecting lens and a suitable substage condenser, 

 without the aid of ground glass, which is sometimes recommended. 



A 1. The extent of the illumination of the field depends : 



1. On the size of the illuminant. 



2. On the effective diameter of the collecting lens ; and 



3. On its distance from the microscope. The farther the col- 



lecting lens is away, the smaller the illuminated area of 

 the field becomes. 



4. On the focal leiigth of the substage condenser. The higher 



the power of the condenser the smaller will be the image 

 of the collecting lens in the field. 



