126 DIFFRACTION. 



the impressed surface displaying all the colours of the ori- 

 ginal body. In fact, the colours of striated surfaces indicate 

 their structure, perhaps more unerringly than any other 

 means : Sir David Brewster has made a very ingenious 

 use of their laws, in investigating the curious and compli- 

 cated structure of the crystalline lens in fishes and other 

 animals. 



(139) There remains another class of phenomena pro- 

 duced by diffraction, which it is important to notice. 



"We have already seen the effects produced, when light 

 diverging from a luminous point is transmitted through a 

 narrow aperture, and received on a screen. But if we 

 vary the experiment, by placing a lens of considerable focal 

 length as the object-glass of a telescope immediately be- 

 hind the aperture, and receive the image on a screen at 

 the conjugate focus, the appearances displayed are altered 

 in a remarkable manner, and differ more widely from those 

 produced in the former case, as the aperture is greater. 

 In fact, the phenomena of diffraction are thus produced with 

 apertures of considerable dimensions, and were observed by 

 Sir William Herschel with the undiminished object-specula 

 of his great telescopes : they are rendered more distinct, 

 however, when the aperture of the telescope is limited by a 

 diaphragm of moderate size. When a star is viewed through 

 a telescope of high power, having its object-lens thus limited, 

 its image is encompassed with a system of diffracted rings 

 slightly coloured, succeeding one another at equal intervals, 

 the diameters of the rings being inversely as the diameters of 

 the apertures. The phenomena vary in a very curious 

 manner when the form of the aperture is changed. Thus, 

 when a triangular diaphragm is substituted for the circular 

 one, the disc of the star appears surrounded by a black ring, 

 from which diverge six rays at equal intervals. 



