MANAGEMENT OF THE MICROSCOPE. 



155 



ference of the Sound-waves. l The colored fringes are produced by the 

 superposition of all these bands. When, again, a small opaque plate of 

 any substance is interposed in the course of the pencil of solar light ad- 

 mitted into a darkened room through a very small hole in a card, or di- 

 verging from the point at which it has been collected by a convex lens of 

 short focus, the shadow thrown by it on the screen will be surrounded by 

 a series of colored fringes, and the shadow itself will be larger than the 

 geometrical shadow. But, further, if a piece of glass be ruled by a dia- 

 mond with parallel lines, some hundreds or thousands to an inch, so as 

 to form what is called a ' grating/ and the narrow beam proceeding from 

 the slit be looked-at through this grating (so held that the direction of 

 its lines is parallel to that of the slit), a number of spectra come into 

 view, ranged at nearly equal distances on both sides of the slit. 2 Now, it 

 is manifest that when a beam of light is made to pass through an object 

 that is being examined Microscopically, the light and shade in the picture 

 seen by the eye must be occasioned by the greater or less transparence of 

 the different parts of that object; and that, wherever there are definite 

 lines or margins sufficiently opaque to throw a definite -shadow, such 



FIG. 116. 



Scale of Gnat showing beaded markings; photographed by Dr. Woodward. 



shadow must be bordered more or less obviously by ' interference ' or 

 ' diffraction ' spectra, especially in the case of objects having strongly- 

 marked lines with very transparent intermediate spaces. There are many 

 objects of great delicacy, in which ' diffraction-bands ' are liable to be 

 mistaken for indications of actual substance; whilst, on the other hand, 

 the presence of an actual substance of extreme transparence may some- 

 times be doubted or denied, through its image being attributed to diffrac- 

 tion. No rules can be given for the avoidance of such errors; since they 

 -can only be escaped by the discriminating power which education and 

 habit confer on the experienced Microscopist. A good example of this 

 kind is afforded by the minute beading presented in the scales of the Gnat 

 and Mosquito (Fig. 116). These scales are composed of a very delicate 

 double membrane, strengthened by longitudinal ribs on either side, those 



1 The colors of thin plates, as seen when the sun shines on a soap-bubble or 

 on a film of oil spread out over a surface of water or when we look at a window 

 through two glasses separated by an attenuated film of air, are familiar exam- 

 ples of 'interference-fringes,' which, when displayed annularly, are known as 

 * Newton's rings.' 



2 Such * gratings ' are now much used in Spectroscopic observation; and afford 

 the best means of determining the wave-lengths of the rays of the several parts 

 -of the spectrum. 



