214 Royal Society. 



valve become separated into two sets, one passing through the 

 thinner depressed portions, the other through the thicker and unde- 

 pressed portions: still both sets enter the object-glass. But on trans- 

 mitting oblique light through the object, one set of the rays will be 

 refracted so as not to enter the object-glass, whilst the other set will 

 gain admission ; thus the two parts, which have differently refracted 

 the rays, will become distinct. If the markings were more delicate, 

 or if the difference between the refractive power of the two portions 

 of the valve were less, both sets would enter the object-glass. But 

 on rendering the light still more oblique, one set would be again 

 excluded by being refracted out of the field Hence it is evident 

 why the angular aperture of the object-glass must be larger as 

 the markings are finer, or the difference between the refractive 

 power of the two portions of tissue is less ; because the obliquity 

 of the light requisite will be very great to cause the exclusion 

 of one set of the rays, and the other set will be too oblique to 

 enter the object-glass unless it be of correspondingly large aperture. 

 This is the explanation of the advantage of oblique light. It has 

 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 ohlique 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 



