ABERRATION OF LIGHT. 105 



We make a narrow opening in a screen, and fix behind this opening a 

 violet glass, penetrable only by red and violet rays. We then place a light, 

 the red rays of which reach the eye of the observer after having passed 

 through the glass and the opening in the screen. If the eye is adapted to 

 the red rays, the violet rays will form a circle of diffusion, and a red point 

 encircled with a violet aureola is seen. The eye may also be brought to a 

 state of refraction, so that the point of convergence of the violet rays is in 

 front, and that of the red rays behind the retina, the diameters of the red 

 and violet circles of diffusion being equal. It is then only that the luminous 

 point appears monochromatic. When the eye is in this state of refraction, 

 the simple rays, whose refrangibility is maintained between the red and the 

 violet rays, unite on the retina. 



There is another kind of aberration of luminous rays of one colour 

 emitted through a hole, which generally only approach approximately to a 

 mathematical focus, in consequence of the properties of refracting surfaces ; 

 it is called aberration of spJiericity. The phenomena are as follows : 



i. We take for our object a very small luminous point (the hole made by 

 a pin in some black paper, through which the light passes), and having also 

 placed before the eye a convex glass, if we are not 

 near-sighted, we fix it a little beyond the point 

 of accommodation, so that it produces on the retina 

 a little circle of diffusion. We then see, instead 

 of the luminous point, a figure representing from 

 four to eight irregular rays, which generally differ 

 with both eyes, and also with different people. 

 We have given the result of M. Helmholtz's ob- 

 servations in fig. 100; a corresponds to the right eye, and b to the left. The 

 outer edges of the lumirious parts of an image, produced in this way by a 

 white light, are bordered with blue ; the edges towards the centre are of a 

 reddish yellow. The writer adds that the figure appears to him to have 

 greater length than breadth. If the light is feeble, only the most brilliant 

 parts of the figure can be seen, and several images of the luminous point 

 are visible, of which one is generally more brilliant than the others. If, on 

 the other hand, the light is very intense, if, for example, the direct light 

 of the sun passes through a small opening, the rays mingle with each 

 other, and are surrounded by aureola of rays, composed of numberless 

 extremely fine lines, of all colours, possessing a ^ g_. 



much larger diameter, and which we distinguish 

 by the name of the aureola of capillary rays. 



The radiating form of stars, and the distant 

 light of street-lamps belong to the preceding 

 phenomena. If the eye is accommodated to a 

 greater distance than that of the luminous point, 



and for this purpose, if the luminous point itself is distant, we place before 

 the eye a slightly convex lens, we see another radiating image appear, 



IOI. 



