34 PRACTICAL PHOTO-MICROGRAPHY. 



delineation. These condensers are usually of three combina- 

 tions, and range to the highest attainable immersion apertures. 

 The best that has ever come under the notice of the writer is 

 the Apochromatic Immersion Condenser of Powell and Lea- 

 land, giving an immersion angle of N. A. 1.4 or even higher. 

 The focal length of this condenser is about one-fifth of an inch, 

 so that in the absence of a bulPs-eye which the writer never 

 uses with this condenser the illuminated field is but small 

 unless the power of objective used be high ; for critical work 

 this shortness of focus is an advantage. The price of achro- 

 matic condensers of high angle is considerable, but their 

 superiority over non-achromatic condensers is great. Messrs. 

 Beck, Swift, and probably all opticians make achromatic 

 condensers to nearly the full extent of the air angle, but one 

 of 140 deg. will be found a very useful condenser where the 

 pocket does not permit of more than a few pounds of expense. 



FIG. 4. IRIS DIAPHRAGM. 



The condenser, as will be more fully shown hereafter, is not 

 so much a device for throwing a blaze of light upon the object, 

 as for (1st) focusing the light at a certain point, and (2nd) 

 modifying the angle and the direction of the light. Accord- 

 ingly the condenser is furnished with various accessories which 

 must be named here, though their precise use must wait till a 

 later time. A set of diaphragms usually accompanies the con- 

 denser, and these have apertures graduated in size from the 

 full aperture of the optical part of the condenser down to very 

 email holes, the smallest of all usually serves for centering. An 

 exceeding great convenience is the Iris Diaphragm, Fig. 4, 



