WHITE-CLOUD ILLUMINATORS. 117 



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, whilst it receives adequate illumination from 

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

 surfaces are distinguished without the least difficulty. 



87. White-Cloud Illuminators. — It being universally admitted 

 that the light of a bright white cloud is the best of all kinds of 

 illumination for nearly every kind of Microscopic inquiry, various 

 attempts have been made to obtain such light from the direct rays 

 either of the Sun or of a Lamp, by what may be called an artificial 

 cloud. Some have replaced the plane mirror by a surface of 

 pounded glass or of carbonate of soda, or (more commonly) by a 

 disk of plaster- of- Paris, the latter being decidedly the preferable 

 method ; but a sufficiently bright light is not thus obtained, unless 

 a Condenser be employed to intensify the illumination of the 

 mirror. Such a Condenser may be most conveniently attached by 

 a jointed arm to the frame which carries the disk, according to the 

 method of Messrs. Powell and Lealand, shown in Fig. 65 ; the 

 frame itself being made to fit 

 upon the Mirror, and to turn Fig. 65. 



with it in every direction. 

 Another very simple, and for 

 many purposes very efficient, 

 mode of obtaining a white-cloud 

 illumination (invented by Mr. 

 Handford) consists in coating 

 the back of a concave plate of 

 glass, like that employed in the 

 ordinary concave Mirror, with 

 white zinc paint, instead of sil- White-Cloud Illuminator, 



vering it ; and then mounting 



this in a frame, which may be fitted (like the plaster-of- 

 Paris disk just described) over the ordinary Mirror. A concave 

 surface of plaster-of-Paris, moreover, may easily be obtained, by 

 casting it when fluid upon the convex surface of such a plate. 

 When a concavity is thus given to the white surface, its perform- 

 ance with low powers is much improved ; but with high powers 

 a special condensation of the light must be adopted, and the 

 arrangement above described seems the simplest that could be 



