182 MANAGEMENT OF THE MICROSCOPE. 



less complete, to the perfectly free transmission of the rays. 

 There are many objects of great delicacy, in which the " diffrac- 

 tion-band" is liable to be mistaken for the indication of an actual 

 substance ; on the other hand, the presence of an actual sub- 

 stance of extreme transparency, may sometimes be doubted or 

 denied, through its being erroneously attributed to the " diffrac- 

 tion-band." 1 No rules can be given for the avoidance of such 

 errors, since they can only be escaped by the discriminative 

 power which education and habit confer. The practised Micro- 

 scopist, indeed, almost instinctively makes the requisite allow- 

 ance for diffraction ; and seldom finds himself embarrassed by 

 it, in the interpretation of the visual appearances which he 

 obtains through a good instrument. Besides this unavoidable 

 result of the inflection of the rays of light, there is a peculiar 

 phenomenon attendant upon oblique illumination at certain 

 angles in one direction ; which consists in the production of a 

 double image, or a kind of overlying shadow, sometimes pre- 

 senting markings equally distinct with those of the object itself. 

 This image, which is not unlike the secondary spectrum formed 

 by reflection from the outer surface of a silvered-glass mirror, 

 has been called the " diffracting spectrum ;" but its origin does 

 not really lie in the diffraction of the luminous rays, since on the 

 one hand it cannot be explained according to the laws of diffrac- 

 tion, and on the other it may be traced to an entirely different 

 cause. An object thus illuminated is seen by two different sets 

 of rays ; those, namely, of transmitted light, which pass through 

 it obliquely from the source of the illumination to the opposite 

 side of the object-glass ; and those of radiated light, which, being 

 intercepted by the object, are given off' from it again in all di- 

 rections. (The latter alone are the rays whereby the images are 

 formed in any kind of "black-ground" illumination, 61, 62.) 

 Two different images will be formed, when the illuminating 

 pencil is very oblique, and the angular aperture of the object- 

 glass is wide ; one of them by the light transmitted to one ex- 

 treme of its aperture, the other by the light radiated to its gene- 

 ral surface ; and one or the other of these images may be stopped 

 out, by covering that portion of the lens which receives, or that 

 which does not receive, the transmitted pencil. This " diftract- 

 ing-spectrum" may be produced at pleasure, in an object illu- 

 minated by direct light and seen with a large aperture, by hold- 

 ing a needle or a horse-hair before the front lens, so as to split 

 the aperture into two parts. 



97. Errors of interpretation arising from the imperfection of 

 the Focal adjustment, are not at all uncommon amongst young 



1 Thus the account given by Prof. Sharpey and the Author, of the structure of Muscu- 

 lar Fibre (Chap. XVIII), has been called in question by observers who had not seen 

 their preparations, on the ground that the " diffraction-band" had not been allowed for. 

 To whatever the appearance in question (Fig. 326) may be due, there cannot be the 

 ^lightest question that it does not arise from diffraction. 



