140 ACCESSORY APPARATUS. 



ject; and thus to illuminate it more and more exclusively by 

 those which meet at the widest angle. In using either of these 

 illuminators, the rays which are made to fall upon them should 

 be parallel, consequently the plane mirror should always be em- 

 ployed ; and when, instead of the parallel rays of daylight, we 

 are obliged to use the diverging rays of a lamp, these should be 

 rendered as parallel as possible, previously to their reflection 

 from the mirror, by the interposition of the "bull's eye" con- 

 denser ( 64) so adjusted as to produce this effect. 



62. For the exhibition of those classes of objects which are 

 suitable for "black ground" illumination, and which are better 

 seen by light sent into them from every azimuth, than they are 

 by a pencil, however bright, incident in one direction only, no 

 more simple, convenient, and efficient means could probably be 

 found, than that which is afforded by the " spotted lens" for low 

 powers, and by the "parabolic illuminator" for powers as high 

 as l-4th or l-5th of an inch focus ; the use of the latter with 

 the highest powers, being rendered disadvantageous by the great 

 reduction in the amount of light, occasioned by the necessity for 

 cutting off of all the rays reflected from the paraboloid, which 

 fall upon the object within the limits of their angle of aperture. 

 One of the great advantages of this kind of illumination consists 

 in this : that, as the light radiates from each part of the object as 

 its proper source, instead of merely passing through it from a more 

 remote source, its different parts are seen much more in their 

 normal relations to one another, and it acquires far more of the 

 aspect of solidity. The rationale of this is easily made apparent 

 by holding up a glass vessel with a figured surface between one 

 eye and a lamp or a window, so that it is seen by transmitted 

 light alone ; for the figures of its two surfaces are then so blended 

 together to the eye, that unless their form and distribution be 

 previously known, it can scarcely be said with certainty which 

 markings belong to either. If, on the other hand, an opaque 

 body be so placed behind the vessel, that no rays are transmitted 

 directly through it, w r hilst it receives adequate illumination from 

 the circumambient light, its form is clearly discerned, and the 

 two surfaces are differentiated without the least difficulty. 



63. Polarizing Apparatus. In order to examine transparent 

 objects by polarized light, it is necessary to employ some means 

 of polarizing the rays before they pass through the object, and'to 

 apply to them, in some part of their course between the object 

 and the eye, an analyzing medium. These two requirements 

 may be provided for in different modes. The polarizer may be 

 either a bundle of plates of thin glass, used in place of the 

 mirror, and polarizing the rays by reflection ; or it may be a 

 "single image" or "Mcol" prism of Iceland Spar, which is so 

 constructed as to transmit only one of the two rays into which a 

 beam of ordinary light is made to divaricate on passing through 

 this substance ; or it may be a plate of Tourmaline, or one of 



