87 



By the addition of the meniscus we obtain greater purity and 

 intensity of illumination, and as the silver reflector is now closed 

 with glass, it is hermetically sealed and permanently protected from 

 dust and damp, and will therefore retain its polish. 



The advantages expected by these arrangements have been 

 realized, the apparatus answering the purpose of an illuminator for 

 all the object-glasses, the objects being seen on a black ground. 



The light most suitable for this method of illumination is lamp 

 or candle-light, the rays of which must in all cases be rendered 

 parallel, by means of a large plano-convex lens or condenser ; the 

 light may then be used direct or reflected from the plane mirror. 

 The object having been adjusted, the illuminator is moved to or fro 

 till the best effect is produced. 



For the purpose of viewing some objects, such as the Naviculee, 

 the circular diaphragm should be slid on the extremity of the 

 apparatus, and revolved till the two pencils of light are thrown most 

 suitably across the object. 



All objects, either transparent or opaque (excepting white), absorb 

 some of the rays of light ; they are rendered visible by that portion 

 which they radiate ; and a predominance of those rays, either 

 primary or compound, of which ordinary white light is composed, is 

 the cause of their various colours, the intensity of which depends on 

 the quantity radiated ; therefore any object tolerably capable of 

 radiating light will be well shown by this method of illumination, 

 and in all their natural colours (if the light thrown on them is 

 achromatic). For these reasons it is probable that it is the radiant 

 rays of light,' coming only from some objects, that we ought to see, 

 and that all the light surrounding, and also that coming directly 

 through, the object, is in some cases not only useless, but detri- 

 mental, for reasons I shall now explain. 



In viewing objects by light transmitted directly through them, we 

 have two sets of rays entering the eye, viz., those emanating from 

 the source of light and those radiating from the object, the imper- 

 fections produced by the former passing through and around the 

 object, mingling with the latter, and preventing them from producing 

 their proper effect. The defects I allude to in direct illumination 

 are caused by the interference of light, the effects of which may be 

 illustrated by the following experiment. Make a hole in the window- 

 shutter of a darkened room, of about a thirtieth of an inch in 

 diameter, upon which fasten a lens, of an inch focus, centrally. 

 When either a transparent or opaque object is held in the beam of 



