USE AND MANIPULATION OF MICKOSCOPE 169 



some workers of repute are of the opinion that no optical system 

 should intervene between the source of light and the sub-stage 

 condenser of the microscope. While for observational purposes 

 this may hold, it becomes a matter of extreme difficulty to do 

 without some such appliance in photographic work. If for no 

 other reason, the pictorial effect would be so much interfered 

 with unless the field is evenly illuminated, that some method of 

 obtaining this desired result becomes almost essential. The 

 necessity for this is more particularly in evidence with a source 

 of light of small area such as the electric arc, or electrical 

 illuminants of the Nernst type. With lime-light, acetylene, or 

 the oil lamp, it is quite possible to work with moderate or high 

 powers without an intervening lens system ; but in any case 

 the exposure is lengthened, and it is by no means clear that 

 any deterioration of the image results from its use. 



For very low-power work the auxiliary condenser may be 

 replaced by a piece of ground-glass, and an enlarged image of 

 the illuminant may be projected on to this ; the ground- glass 

 in effect therefore becomes the source of light. The conditions 

 with higher powers are that either the image of the source of 

 light is enlarged by using an auxiliary lens so that it covers a 

 larger proportion of the field, or the position of the radiant is 

 so adjusted that the aperture of the lens itself is what is actually 

 f ocussed in the field of view the image therefore in the field of 

 view appearing as a luminous disc. 



Perhaps the most serious practical objection, and one that 

 requires to be carefully guarded against, is that any intervening 

 lens, unless exactly and properly adjusted, may, under certain 

 circumstances, reduce the working aperture of the objective ; 

 and it is against this contingency more than any other that 

 the worker is warned. 



The simplest lens of this type is of course the bull's-eye, 

 of which a description has already been given, and which is only 

 suitable for use with low powers. A development of the bull's- 

 eye system is the more highly corrected lens system devised 

 by Mr. E. M. Nelson, and usually known by his name. This 

 lens has already been described under the section devoted 

 to such apparatus in Chap. IV, p. 70, as well as the Watson- 

 Conrady system, which is still better and the corrections of 

 which satisfy the requirements of the most critical worker. 



