89 



of transparent objects being better seen where the direct light is 

 entirely or partially intercepted, for I should observe that some 

 objects, especially those of a dark colour, are better shown on a 

 ground either coloured or slightly illuminated. This circumstance 

 is provided for in the reflector by turning out the bottom of the dark 

 well, so that patches of tissue-paper, more or less diaphanous or 

 variously coloured, may be used for the back-ground of the object. 

 I shall now explain why, on test-objects and the internal mechanism 

 of Infusoria, markings can be seen by this method which are invisible 

 by the ordinary way of illumination. 



There is but little doubt that the markings on most of the test- 

 objects are either depressions or projections. By the usual direct 

 light all parts of the surface of an object, whether raised or depressed, 

 are illuminated with nearly equal intensity, and the strong direct 

 light partially destroys delicate colours ; consequently there is a want 

 of contrast. 



The effect of an angular pencil of rays of a hundred and seventy- 

 five degrees, with the central ones stopped, is, that there is a greater 

 relative amount of light thrown on these prominences, as they inter- 

 cept the largest portion of the circular marginal or most oblique rays 

 near the apex of the reflector (which are thence the most effective), 

 leaving the base of the prominence in comparative shadow; conse- 

 quently the very markings we wish to see are the most strongly 

 illuminated. 



The different organs in the interior of an animalcule may be much 

 of the same transparency and colour, and yet possess a different 

 refraction, according to their density ; therefore the illumination 

 should, if possible, be so arranged as to take advantage of this cir- 

 cumstance. Now direct light will pass through these transparent 

 membranes in straight lines, without being affected by their various 

 refractive powers. The effect of lateral or oblique light on such 

 tissues is, that the rays are more refracted according to their incli- 

 nation, and proportionate to the various densities of the medium ; 

 the most refractive structure transmitting the greatest quantity of 

 light, and is in consequence more illuminated, and this reason is 

 somewhat confirmed by the circumstance of this circular or lateral 

 illumination showing the structure of some objects which, from slight 

 variation in density, are invisible except by the polariscope. 



I have thus endeavoured to place my ideas of microscopic illumi- 

 nation before the Society in an intelligible point of view, though 

 perhaps not very systematically, as to treat the subject fully would 



TRANS. MIC. SOC. VOL. III. N 



