THE PARABOLIC BEFLECTOR. 63 



ject having been adjusted, the illuminator is moved to and fro till the 

 best effect is produced. For the purpose of viewing some objects, such 

 as the navicula, 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, with the exception of 

 white, absorb some of the rays of light, and are rendered visible by 

 that portion which they radiate : a predominence 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 upon 

 the quantity radiated ; therefore any object at all capable of radiating 

 light will be well shown by this mode of illumination, and, if the light 

 thrown on them is achromatic, in all their natural colours. 



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 imperfec- 

 tions produced by the former passing through and around the object 

 mingling with the latter, and preventing them from producing their 

 proper effect.'' 



As the method of illuminating microscopic objects by means of a 

 large angular pencil of light, having the central rays obscured, is of 

 recent introduction, we shall mention a few instances where transparent 

 objects are shown, under similar circumstances, with perfect or im- 

 proved definition. 



The lateral mode of illumination will be found to possess peculiar 

 advantages in the examination of test-objects and the internal me- 

 chanism of infusoria. The markings on most of the test-objects are 

 either depressions or projections by direct light : all parts of an object 

 are illuminated with equal intensity, and delicate colours are in great 

 part destroyed, consequently there is a want of contrast. The effect of 

 an angular pencil of rays of 175°, with the central ones stopped, is, 

 that there is a greater relative amount of light thrown on these pro- 

 minences, as they intercept the largest portion of the marginal rays 

 near the apex of the reflector, leaving the base of the prominence 

 in comparative shadow, consequently the markings we wish to see are 

 the most strongly lighted. The different organs in the interior of an 

 animalcule may be much of the same colour and transparency, and yet 

 possess a different refraction, according to their density. Direct light 

 wiU pass through these transparent membranes in straight lines with- 

 out being affected by their various refractive powers. The effect of 

 lateral or oblique light on such tissues is, that the rays are more re- 



