The Illumination of Objects. 53 



means of Mr. Collins' " Graduating Diaphragm/' or by some 

 other contrivance. 



Those who have contented themselves with ordinary 

 modes of illumination will be surprised at the variety of 

 ■effects to be produced with the same slide by different 

 methods of lighting it up. As a mere source of amusement, 

 nothing can be pleasanter than to spend an evening or two in 

 rtrying experiments of this kind. Very transparent objects 

 should be tried with different- sized pencils of light, composed 

 «af rays either very divergent, or nearly parallel, and with all 

 grades of intensity. Highly refractive objects, which are at 

 the same time good reflectors, such as many of the polyparies 

 of compound polyps (Sertularia, etc.), or plyzoaries (as the 

 abodes of compound associated polyzoa have been called), are 

 very fit for experiments, with varying angles of opaque illu- 

 mination obtained by bulls'-eyes, lieberkuhns, or side reflectors, 

 and also for other experiments with spot lenses, parabolic 

 illuminators, or the dark ground stops of Webster's condenser. 

 All the large diatoms, hairs of animals mounted in Canada 

 balsam, parts of insects, etc., should be treated in the same 

 way. Many objects are nearly equally fit for transparent, 

 opaque, and dark ground illuminations, and, as we have before 

 remarked, an inferior mode of illumination often brings out 

 some specialty which a better mode fails to show. 



In all these experiments it must be remembered that the 

 appearance produced may vary greatly, according to the angle 

 at which the illuminating pencil reaches the object. Even 

 with the same mode of illumination, dark ground, transparent, 

 or opaque, projections or depressions will be seen or not seen, 

 in proportion to the skill which the angle of an illumination is 

 managed. An object may look flat with one illumination, and 

 be seen to have a varied contour the moment another is 

 .employed. 



