THE OPTICAL EQUIPMENT 73 



combination (Fig. 21 b). Lens I is brought so close to the 

 source of light that the latter is approximately in the focal 

 point of the lens and projects therefore in the direction 

 of the microscope a parallel beam. Lens II is now placed 

 in such a position as to receive this parallel beam, collecting 

 the_* light and projecting an image of the radiant on to 

 the diaphragm of the sub-stage condenser of the microscope. 

 In this case the projected image is smaller than where 

 lens I only is used, and consequently the aperture of the 

 sub-stage condenser is somewhat reduced, but as the 

 arrangement is for medium powers only, this is a matter of 

 no moment. 



For low magnifications with lenses from | to 1 inch, where 

 a relatively large area has to be illuminated, lenses I and III 

 are used in conjunction (Fig. 21 c). These lenses are of such 

 focal length that the image of the source of light projected 

 on to the iris- diaphragm of the microscope is of the same 

 size as the radiant ; consequently it is only useful for the 

 powers indicated, owing to the sub-stage condenser being but 

 partially utilised. It is important in working any of these 

 arrangements to have an iris-diaphragm to cut off extraneous 

 or stray light, and it should always be placed approximately 

 in a position as shown in Fig. 21. Such a diaphragm is 

 advisable in any case, as it can be regulated to allow the 

 field of view of the objective to be just filled and no more ; 

 so that extraneous light, from whatever cause it may arise, is 

 excluded, and the effect of any internal reflections is elimin- 

 ated. It will be found that by closing this iris- diaphragm 

 and projecting an image of it in the field of view of the micro- 

 scope, the illumination will be quite satisfactory, and will in 

 general give results equal to any critical illumination obtained 

 by the method in which the image of the radiant is projected 

 on to the object. 



In practice a rather long distance is required between the 

 radiant and the microscope, and where several pieces of appa- 

 ratus have to be in perfect alignment it is advisable to carry 

 them on a triangular metal bar, or on some similar arrange- 

 ment where perfect alignment can be maintained. The 

 saddle stands made by Messrs. Zeiss for this purpose are in 

 every respect satisfactory, but a method of supporting lenses 



