166 ERRORS OF INTERPRETATION. 



almost instinctively makes the requisite allowance for diffraction ; 

 and seldom finds himself embarrassed by it in the interpretation of 

 the visual appearances which he obtains through a good instru- 

 ment. 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 presenting markings equally distinct with those of the 

 object itself. This image, which is not unlike the secondary image 

 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 the radiated light, which, being 

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

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

 formed in any kind of ' Black-Ground ' illumination, §§ 84, 85.) 

 Hence 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 extreme of its 

 aperture, the other by the light radiated to its general surface ; 

 and one or the other of these images may be stopped-out, by cover- 

 ing that portion of the lens which receives, or that which does not 

 receive, the transmitted pencil. This ' diffracting spectrum ' may 

 be produced at pleasure, in an object illuminated by direct light 

 and seen with an Objective of large angular aperture, by holding 

 a needle or a horsehair before its front lens. 



127. Errors of interpretation arising from the imperfection of 

 the Focal adjustment are not at all uncommon amongst young 

 Microscopists. With lenses of high power, and especially with 

 those of large angular aperture, it very seldom happens that all 

 the parts of an object, however minute and flat it may be, can be 

 in focus together ; and hence when the focal adjustment is exactly 

 made for one part, everything that is not in exact focus is not only 

 more or less indistinct, but is often wrongly represented. The 

 indistinctness of outline will sometimes present the appearance of 

 a pellucid bonier, which, like the diffraction-band, may be mistaken 

 for actual substance. But the most common error is that which is 

 produced by the reversal of the lights and shadows resulting from 

 the refractive powers of the object itself : thus, the bi-concavity of 

 the blood-disks of Human (and other Mammalian) Blood occasions 

 their centres to appear dark when in the focus of the Microscope, 

 through the dispersion of the light which it occasions ; but when 

 they are brought a little within the focus by a slight approximation 



