172 



THE MICROSCOPE. 



Mr. Nelson recommends as the most useful of his 

 diaphragms those represented in fig. 104, in which a 

 may be regarded as a type shape for one pencil of light, 

 and b for two, at right angles. The superposition of 

 stops c will cut off more or less of the central light, 

 d will stop out more or less of the peripheral zone ; 

 while e is a combination intended to utilize the most 



FIG. 104. Nelson's Diaphragms. 



oblique pencil required for the resolution of fine lined 

 objects. A variety of discs of the forms c and d may 

 be used ; any of which, dropped into a metal holder 

 with an inner ring made deep enough to receive two or 

 three, which when in place can be rotated by a milled 

 edge, or moved out of the axis by the handle. 1 



Dark Field Illuminators. To Mr. F. H. Wenham's 

 the microscope is deeply indebted for many valuable 

 improvements ; not the least important being the dark- 

 field or parabolic illuminator, invented 

 in 1851. The operation of the para- 

 bolic condenser (fig. 105) depends for 

 its action on rays thrown on the object 

 at an angle extending beyond that 

 known as the aperture of the object- 

 glass, and which otherwise would be 

 lost ; consequently, as the source from 

 which the light comes is without the 

 range of the pencil of rays of the ob- 

 jective, the field must be dark ; but if 

 an object possessing a partial opacity is 

 placed exactly in focus, it becomes brilliantly luminous 

 by means of these rays. Dark-ground illumination is 

 not suitable for very transparent objects that is, unless 

 there is a considerable difference in their refraction, or 

 they are pervaded by air-cells. 



One very remarkable example of this fact may be 



(1) Journal of the Royal Microscopical Society, Vol. IV., p. 126 (1881). 



