80 THE MICROSCOPE. 



revolved till the two pencils of light are thrown most 

 suitably across the object. 



" 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, I shall mention 

 a few instances where transparent objects are shown, under 

 similar circumstances, with perfect or improved definition. 

 The lateral mode of illumination will be found to possess 

 peculiar advantages in the examination of test-objects and 

 the internal mechanism 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 prominences, as they intercept the 

 largest portion of the marginal rays near the apex of the 

 reflector, leaving the base of the prominence in compara- 

 tive 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 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 inclination, and 

 proportionate to the various densities of the medium, the 

 most refractive structure transmitting the greatest quantity 

 of light, and being in consequence the most illuminated ; 

 and this reason is somewhat confirmed by the circumstance 

 of lateral illumination showing the structure of some 

 objects which, from slight variation in density, were 

 invisible, except by the use of polarised light." Mr. 

 Shadbolt has since modified this reflector, which he deno- 

 minates "a sphero -annular condenser:" it has superior 

 reflecting arrangements, with less liability of derangement, 

 and is constructed of a solid cylinder of glass terminating 

 above in a solid cone, the surface of which has the form 

 of a parabola, and replaces the silver reflecting surface. 



