74 Intelligence and Miscellaneous Articles. 



satisfaction of knowing that light which is barely intense enough 

 to produce a transient impression upon the retina, will, in time, pro- 

 duce a strong impression upon a sensitive photographic surface. I 

 should however have obtained nothing worth preserving, had not 

 Professor Wheatstone, to whom I mentioned the results of my expe- 

 riments, kindly allowed me to use his magnificent tourmalines and 

 crystals. On placing these in the position above mentioned, I could 

 perceive a very visible image on the focusing glass, without the sun, 

 and, by a few preliminary trials found that even with the diffused 

 light of the sky, collodion would take a good impression with from 

 half an hour to two hours' exposure, varying with the size and nature 

 of the crystal employed. The camera should be placed on the ground 

 and pointed upwards ; in this position the collodion plate will keep 

 for more than three hours, if it be sufficiently level to retain the liquid 

 on its surface. [I find that when kept in this manner its sensi- 

 tiveness is very little impaired.] The best light is from a rather cloudy 

 sky, bright but without sun ; the camera should be turned a quarter 

 round every two or three minutes to prevent the too unequal action 

 of the light. 



The figures produced in unannealed glass under the same circum- 

 stances presented no difficulty in copying. The piece of glass, set 

 in a black opake frame, was placed upright in front of a black glass 

 reflector, arranged so that the diffused light from the sky, falling upon 

 it, might be reflected horizontally at the proper angle and pass through 

 the piece of unannealed glass. The photograph was taken in a camera 

 placed opposite, having a tourmaline or Nichol's prism fastened in 

 front of the lens ; by rotating which, the proper figure was obtained 

 on the ground-glass. 



My first copies were taken on collodion, iodized in the usual 

 manner ; they presented a far greater number of rings both in calc 

 spar and nitre, and extending much further than could be seen in the 

 polariscope : while in the latter I was not able to perceive more than 

 eight or nine riugs, upwards of fifty could sometimes be counted on 

 the photograph, showing in a remarkable manner the greater extent 

 of the rings formed by the more refrangible rays over those formed 

 by the visible ones. 



A few weeks ago on mentioning this circumstance to Professor 

 Wheatstone, he suggested that I might acquire some valuable infor- 

 mation by adopting Sir John Herschel's proposition of substituting 

 bromide for iodide of silver, and cutting off the chemical rays with 

 sulphate of quinine ; thus obtaining an effect due to the light alone. 

 I immediately made the following experiments on this subject: 

 Having first satisfied myself that several inches in thickness of quartz 

 obstructed none of the chemical rays, I made a small cell of about 

 one ounce capacity, the two sides consisting of flat plates of quartz, 

 and the distance across the cell from one side to the other being one 

 inch and a half: this I filled with a solution of sulphate of quinine 

 in dilute sulphuric acid, containing 25 per cent, of the sulphate, and 

 roughly measured the distance to which the invisible rays penetrated, 

 by concentrating a ray of sunlight on it by means of a lens ; the 



