THE OPTICAL EQUIPMENT 51 



use of objectives as sub-stage condensers is practically 

 unnecessary. There is the further objection that high-power 

 lenses are of too short focus to obtain critical illumination 

 through an ordinary object-slide. 



Condensers which have been termed apochromatic have 

 been brought out by one or two makers. The use of this 

 name would not appear to be strictly justifiable, and probably 

 arises from the fact that fluorite, and other glasses similar to 

 that used in the construction of apochromatic lenses, have been 

 utilised. As these condensers do not equal the correction of 

 the apochromatic lenses, for reasons that need not be fully 

 entered into here, they are really not entitled to be so called. 

 At the same time they are usually extremely good, and their 

 performance is equal to that of a high-class achromatic con- 

 denser at its best. 



It may be taken as an axiom that the illumination of an 

 object should be effected with as large a solid cone of light as 

 possible. No objective except when used for the observation 

 of preparations such as bacteria or similar objects will stand 

 illumination with a cone of light equal to its own numerical 

 aperture. The effect of too large a cone is easily seen, as the 

 object appears to be flooded in light and there is a general lack 

 of contrast over the whole field. When this is the case the 

 sub -stage iris -diaphragm should be contracted until the image 

 appears to possess greater contrast and until fine details are 

 clearly defined. If this process of narrowing down the cone of 

 light is carried too far, diffraction-fringes will appear round the 

 components of the structure of the object ; and in no case 

 should this point be reached. The result of narrowing the cone 

 of light by reducing the aperture of the condenser is in effect 

 the same as that obtained by reducing the aperture of the 

 objective. If carried to excess, the size of the diffraction discs, 

 or what are described by Mr. J. W. Gordon as antipoints, 1 are 

 so increased as to overlap ; serious interference to the definition 

 then results. This will be better appreciated if it is remembered 

 that where broad beams of light are passing through an object, 

 the amount of diffracted light in relation to the whole is not 

 large, and it is not therefore sufficient to interfere with the 

 general definition. Where a very narrow beam is passing 



1 Principles of Microscopy, by Sir A. E. Wright, p. 100. 



E2 



