DIFFRACTION. 119 



A plate of metal is perforated with a small round hole, 

 about the j\yth of an inch in diameter, around which, at the 

 distance of ahout J or ^ of an inch, is a circle of smaller 

 holes. The flame of a lamp is then placed immediately 

 behind the aperture, and the luminous point viewed through 

 the substance to be examined. A ring or halo will be seen 

 surrounding the aperture; and by moving the substance 

 backwards and forwards on a graduated scale, this ring may 

 be brought to coincide with the circle of small holes pierced 

 in the plate. The distance from the aperture is then read 

 off on the scale, and varies obviously in the inverse ratio of 

 the angular diameter of the spectrum ; but the diameters 

 of the particles vary also in the same inverse ratio, so that 

 the distance on the scale at once becomes a measure of these 

 diameters. In this manner Young compared the diameters 

 of a great number of very minute substances, such as the 

 fibres of the finest wools, the globules of the blood, &c. The 

 instrument itself he called the Eriometer. 



(132) In the case last mentioned we have supposed the 

 intervals of the fibres, or fine wires, to be much greater than 

 their thickness; in which case the phenomenon depends mainly 

 on the diameter of the opaque fibre. When the intervening 

 apertures are very small, the effect is influenced by their 

 magnitude, and assumes a different character. Thus, if a 

 grating be formed, by stretching a wire between two fine 

 screws of equal thread, and if this grating be held in the 

 beam diverging from a luminous point, we shall observe,, 

 on either side of the direct image, a series of spectral images 

 richly coloured with all the prismatic tints ; the spectra in- 

 creasing in breadth, and diminishing in intensity, as they 

 recede from the centre. 



These phenomena are seen to most advantage by means 

 of a telescope adjusted to the luminous origin. The grating 



