On the Refraction of Light. 355 



able distance from the prism, which, considering the divergent pow- 

 er of the lens beyond its focus, should be much nearer than is re- 

 quisite for examining the common spectrum. The prism employed 

 is very liable to be broken, from the quantity of heat concentrated 

 on it by the lens. 



The luminous pyramids before spoken of are probably images of 

 the prism, which pass in minature through the foramina in the leaden 

 plate, and augment in magnitude as their distance from the lead in- 

 creases ; for when, instead of using the homogeneous rays issuing 

 from a prism, we place red or blue glass in the aperture through 

 which the solar beam is admitted, and allow the rays to pass through 

 holes in a plate of lead ; on receiving the images on paper at some 

 distance from the metal, we shall find them to be perfectly circular. 



It has been maintained by a few of those who have written on the 

 inflection of light, that some of the calorific rays are more diffrangi- 

 ble than others, and as a corollary to this, it follows that differently 

 colored rays, after passing through punctures of equal diameters will 

 form figures whose bases, if received on a white surface, will appear 

 of different sizes. To ascertain the truth of this position, the following 

 experiments were performed. 



A plate of red glass was placed in the aperture in the window 

 shutter ; and at the distance of a couple of feet from it, was fixed a 

 sheet of lead, having in it a round hole of a line in diameter. The 

 boundaries of the base of the cone formed by the rays, were accu- 

 rately marked with a crayon on a sheet of white paper, at the dis- 

 tance of twelve feet from the leaden plate. On substituting blue for 

 red glass in the window shutter, the base of the blue cone was found 

 to occupy precisely the same space on paper as that of the red onOo 

 Yellow and violet glasses were employed, and the cones which they 

 formed were equal to those produced by red and blue. It is to be 

 remarked that these monochromatic cones do not differ in magnitude 

 from the cone formed by compound solar light. The colored fring- 

 es observed round the base of the cone of white light, cannot of 

 course be observed around the base of the monochromatic cone ; 

 but instead of them, appear rings of different intensities of color : 

 towards the centre: of the base the color is bright but dilute, and 

 dark but concentrated around the circumference. 



Lest the equality in the size of the cones might be supposed to 

 arise from any difference or imperfection in the colored media, I 

 receive^ the analyzed rays, emerging from a prism, on a sheet of 



