nELATlVE TO PHYSICAL OPTICA. 



647 



there are strong reasons for believing the co- 

 louring particles of natural bodies in general 

 to be incomparably smaller than this ; and it 

 is probable that the analogy, suggested by 

 Newton, is somewhat less close than he ima- 

 gined. The light reflected by a plate of air, 

 at any thickness nearly corresponding to the 

 1 1 th red, appears to the eye to be very 

 nearly white; but, under favourable circum- 

 stances, the 1 Ith red and the neighbouring 

 celours may still be distinguished. The light 

 of some kinds of coloured glass is, pure red; 

 that of others, red wiUi a little green : some 

 hitercept all the light, except the extreme 

 red and the blue. In the blue light of a 

 candle, expanded by the prism, the portions 

 of each colour appear to be narrower, and 

 the intervening dark spaces wider, than in 

 the analogous spectrum derived from the 

 light reflected from a thin plate. Perhaps 

 their origin may have some resemblance to 

 that of the different harmonics of a single 

 vibrating substance. The light of burning al- 

 cohol appears to be green and violet only. 

 The pink dye sold in the shops, which is a 

 preparation of the carlhamus, aflbrds a good 

 specimen of a yellow green light regularly 

 reflected, and a crimson probably produced 

 by transmission. 



VI. EXPERIMENT ON THE DARK RAYS OF 

 UITTER. 



Exper. 6. The existence of solar rays ac- 

 companying light, more refrangible than 

 the violet rays, and cognisable by their che- 

 mical effects, was first ascertained by Mr. 

 Ritter: but Dr. Wollaston made the same 

 experiments a very short time afterwards, 

 without having been informed of what had 

 been done on the Continent. These rays 



appear to extend beyond the violet rays of 

 the prismatic spectruin, through a space 

 nearly equal to that which is occupied by the 

 violet. In order to complete the comparison 

 of their properties with those of visible light, 

 I was desirous of examining the effect of 

 their reflection from a thin plate of air, ca- 

 pable of producing the well known rings of 

 colours. For this purpose, I formed an 

 image of the rings, by means of the solar 

 microscope, with the apparatus which I have 

 described in the Journals of the Royal Insti- 

 tution, and I threw this image on paper 

 di])ped in a solution of nitrate of silver, placed 

 at the distance of about nine inches from the 

 microscjpe. In the course of an hour, por- 

 tions of three dark rings were very distinctly 

 visible, much smaller than the brightest rings 

 of the coloured image, and coinciding very 

 nearly, in their dimensions, with the rings of 

 violet light that appeared upon the interposi- 

 tion of violet glass. I thought the dark rings 

 were a little smaller than the violet rings, but 

 the difference was not sufficiently great to be 

 accurately ascertained ; it might be as much 

 »s ^or ~'s of the diameters, but not greater. 

 It is the less surprising that the difference 

 should be so small, as the dimensions of the 

 coloured rings do not by any means vary at 

 the violet end of the spectrum, so rapidly as 

 at the red end. For performing this experi- 

 ment with very great accurac)', a heliostate 

 would be necessary , since the motion of the 

 sun causes a slight change in the place of the 

 image ; and leather, impregnated with the 

 muriate of silver, would indicate the eflfect 

 with greater delicacy. The experiment, 

 however, in its present state, is sufficient to 

 complete the analogy of the invisible with 

 the visible rays, and to show that thev are 



