155 



the object which demands attention, and should not be 

 disregarded. 



To those unprovided with the aforementioned prism, and 

 indisposed to incur the expense (about $20), I would recom- 

 mend, as the next best adaptation, the use of an achromatic 

 converging lens, of a focus of from 1 1 to 2 inches, interposed 

 between the mirror and the stage, carefully centred with the 

 optical axis of the instrument, and used with the plane 

 mirror. A double combination of two plano-convex non- 

 achromatic lenses of like focus, similarly placed, is advan- 

 tageous, but will not equal the achromatic combination. 



In oblique, or as usually termed .by Continental micro- 

 scopists eccentric illumination, we have to consider, not only 

 light as radiated from the object, together with the refractions 

 to Avhich the pencils of rays are subjected in their passage 

 through the combinations of the objective, but also are 

 compelled to bear in mind the effects of shadow and relief, 

 as influenced by the parallelism or c6nvergence, as the case 

 may be, of such illuminating pencil. A beam of light falling 

 obliquely upon an object causes an obscuration, darkness, or 

 shadow (technically termed umbra), to be formed upon the 

 opposite side of the object, the depth and intensity of which 

 dej)end ceteris paribus upon the intensity of illumination. 

 Now, it is found that the line of demarcation between the 

 shadow and light is not a marked, well-defined line, but that 

 a lesser shadow, termed penumbra, shades or tones down 

 the separation. This penumbra is greatest, and in just so 

 far lessens the size of the shadow, when a convergent i^encil 

 of light is brought to bear upon the object; and, therefore, 

 a priori, we would infer that illumination by parallel rays 

 would be preferable for microscopic purposes. 



In fact, Mr. J. B. Reade, President of the Royal Micro- 

 scopical Society of London, has but only recently published 

 a series of articles claiming the perfection of microscopical 

 illumination to consist in the employment of parallel rays. 

 His results, hoAvever, are undoubtedly due — 1st, to the use 

 of a reflecting prism in place of the mirror, whereby he loses 

 or avoids the secondary reflection always present on the use 

 of the mirror, and which is so antagonistic to true definition ; 

 2ndly, in having been accustomed undoubtedly to the use of 

 the accessory known as the achromatic condenser, until he 

 himself invented and, if I am rightly informed, patented a 

 far Avorse device, viz. the kettledrum illumination. Now he 

 informs us that such accessories are wrong in both principle 

 and use, and can hardly find Avords sufficiently expressive 

 of the great superiority of his present mode of illumination 



