63 



no peculiar power of rendering objects distinct, as has sometimes 

 been believed, and the following experiment, supposed to show such 

 peculiar power, is really to be explained on different grounds. A 

 piece of net, or some similar texture, is placed behind a hole made in 

 a window- shutter, and when thus viewed, the fibres are not well 

 seen ; but when the texture is moved on one side, they become very 

 distinctly visible, and this has been erroneously attributed to the 

 illumination by oblique light ; whereas the increased distinctness in 

 the lateral position is owing principally to the circumstance that 

 the object is then viewed on a dark instead of a white ground as in 

 the first instance ; although it is also true that in this position the 

 oblique rays, being reflected in large numbers from the fibres into 

 the eye, contribute to the distinct vision of the object when viewed 

 as it then is upon a dark ground. 



The most difficult point has been to explain, how an object- 

 glass of large angular aperture will render markings evident, 

 which were not visible under an object-glass of smaller aperture ; be- 

 cause it would naturally be imagined that the larger aperture would 

 admit both sets of rays, one of which was excluded by the ob- 

 ject-glass of smaller aperture. The difficulty vanishes when it is re- 

 collected that the additional rays admitted by the object-glass of larger 

 aperture are more oblique ; hence one set of these rays will be re- 

 fracted from the field of the microscope, whilst the other set will 

 enter the object-glass and will illuminate the more highly refractive 

 parts of the object ; thus the two kinds of differently refractive struc- 

 ture become distinctly separated, one appearing dark, the other lumi- 

 nous ; in fact, by means of the additional rays admitted by the larger 

 aperture we illuminate more highly one part of the object whilst the 

 illumination of the other is not increased. In short, the object is 

 illuminated, first, by rays corresponding to those admitted by an 

 object-glass of small aperture ; and, secondly, by the additional rays 

 admitted by the object-glass of larger aperture. The first set not 

 being sufficiently oblique, no part of them is refracted beyond the 

 angular aperture of the object-glass ; the second, being more oblique, 

 are refracted out of the field by certain parts of the object and not 

 by others, and thus contribute to render its different parts distin- 

 guishable by contrast of darkness and illumination. The first set of 

 rays, by illuminating all parts of the object, tend to diminish this 



