POWER OF SHORT SPACES. 35 



taper. And if an opening of given shape were substituted for 

 the opaque object, as many illuminated spaces would be pro- 

 jected on the wall instead of the shadows. 



This is effected on a small scale in the diascope, where small 

 perforations which admit the light are substituted for the 

 tapers, transparent designs on glass for the object, and the 

 retina of the eye for the screen on the wall. 



Beautiful combinations on a large scale might be projected 

 on an extended surface by the multiplication of shadows, but 

 it is not our purpose to examine bodies at ordinary distances. 



Hitherto but few experiments have been instituted for the 

 purpose of showing what kinds of images are produced with- 

 out a lens by bodies held close in front of the eye. It is not 

 likely, therefore, that all the necessary conditions shall be de- 

 vised until more care and attention shall have been bestowed 

 on this interesting branch of optics. Those which have been 

 mentioned in the former papers, and are resumed in this, may 

 possibly prove sufficient to provoke inquiry, inasmuch as they 

 are based on legitimate conclusions from the known laws of 

 optics, and are confirmed by experiments. 



Small circular, as well as elongated openings for the trans- 

 mission of light were used by Grimaldi, Newton, Fresnel, and 

 Frauenhofer for investigating the phenomena, which light pro- 

 duces, when passing near the edges of bodies, a branch of 

 optics which is called the inflexion^ or the diffraction of light. 



A divergent beam of light was obtained by causing the 

 sun's rays to pass through one of these apertures, and it was 

 ascertained that the shadows of all bodies whatever, held in 

 this light, were not only surrounded, but encroached on by 

 fringes of colours. 



The experiments themselves were instituted for the purpose 

 of ascertaining the magnitude, form, colour, and number of 

 such fringes, when examined either by common or by homo- 

 geneous light. 



The aperture, moreover, was held six feet or upwards from 

 the eye, and the fringes were seen either by throwing them on 

 a smooth white surface, where they could be examined with 

 the naked eye, or by looking at them with a magnifying glass, 

 in which case their peculiarities could be more carefully in- 

 vestigated. 



According to Sir David Brewster, this curious property of 

 light was ably and successfully investigated by Fresnel, but 

 the finest experiments on this subject are those of Frauenhofer.* 



* See Sir David Brewster's ' Optics,' Cabinet Cyclopaidia ; also Herschel's 

 ' Treatise on Light,' ^ 735 ; also Edinburgh Cyclopa3dia, art. 'Optics,' vol. xv., 

 p. 556 ; also ' Elements of Natural Philosophy,' by Bird and Brooke. 



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