Experiments in Photography. 329 



refrangible than the violet rays, and cognisable by their chemical 

 effects, was first ascertained by Mr. Hitter; but Dr. Wollaston 

 made the same experiments a very short time afterwards, without 

 having been informed what had been done on the Continent. 

 These rays appear to extend beyond the violet rays of the pris- 

 matic spectrum, 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 reflexion from a thin plate of air 

 capable of producing the well-known rings of colours. For this 

 purpose I formed an image of the rings, by means of the solar 

 microscope, wath the apparatus which I have described in the 

 Journals of the Royal Institution, and I threw this image on 

 paper dipped in a solution of nitrate of silver, placed at a distance 

 of about nine inches from the microscope. In the course of an 

 hour, portions 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 interposition 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 accu- 

 rately ascertained ; it might be as much as ^^(jth or Tf^gth of the 

 diameters, but not grcatej'. 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 experiment 

 with very great accuracy a heliostate would be necessary, since 

 the motion of the sun causes a slight change in the place of the 

 image, and leather impi-egnated with the muriate of silver would 

 indicate the effect with greater delicacy. The experiment, how- 

 ever, in its present state is sufficient to complete the analogy of 

 the invisible with the visible rays, and to show that they are 

 equally liable to the general law (of interference), which is the 

 principal subject of this paper." 



It detracts nothing from the greatness of Dr. Young to say, 

 that although^ the philosophy of this experiment is permanent 

 truth, yet the spectral image of it soon faded away. Photo- 

 graphy was not then, at the beginning of the century, an art as 

 permanent as it is elegant and useful. Little was wanted to 

 make it so, but it hung fire for nearly fifty years, till Talbot 

 supplied that little. 



I have just learnt from Admiral Smyth, that his friend 

 Dr. Peacock, the Dean of Ely, has for the last seven years been 

 engaged on a life of Dr. Young; and when the work appears, we 

 shall have a more accurate knowledge of the man who was un- 

 questionably the Newton of his day. Like his illustrious pre- 



