NATURAL PHILOSOPHY. 157 



appearance of black or white. On a bright day, with white clouds in the sky, 

 the rings were colored respectively a light yellowish-green, two different 

 shades of purple, and a pink. He also cut a spiral figure of card, the revo- 

 lution of which produced most beautiful colors in those parts offering- certain 

 proportions of black and white. The position, whether horizontal or verti- 

 cal, in which the disks revolve, does not affect the result, and the colors can 

 be reflected on a white screen ; thus proving that they do not result from any 

 illusion caused by the dazzling motion of the eye. From the experiments 

 Mr. Smith concludes that light is simple and not compound, and that the 

 phenomena of prismatic refraction and polarization of light must be ex- 

 plained upon hypotheses altogether different from those of Newtou. 



WOODWARD'S SOLAR CAMERA. 



The Solar Camera, invented by Mr. "Woodward, of Baltimore, Md., is, as its 

 name implies, an adaptation of the principle of the solar microscope to the 

 ordinary camera, for the pui-pose of obtaining a light sufficiently strong to 

 be used for enlarging small photographs. Mr. Woodward is an artist by 

 profession, and it often occurred to him that if he could get sufficiently en- 

 larged copies of ordinary photographs to paint over canvas, it would be of 

 great advantage; and, following up the idea, he has produced the invention in 

 question. The condenser is placed in such a position that the point where 

 the rays of light cross, answers to the diaphragm ordinarily used; and by 

 this plan loss of light is avoided, and the image is free, or nearly so, from 

 spherical aberration. Another advantage of the solar camera is, that the 

 pictures can be printed direct upon the sensitive paper, thus avoiding the 

 necessity of making a second negative, or of developing the paper pictures. 

 Mr. Woodward generally uses ammonio-nitrate paper, and sometimes albu- 

 menized paper. 



The powers of the Solar Camera have been put to a severe test in the United 

 States Coast Survey. Thus, it was desired to ascertain how far it would be 

 practicable to enlarge small copies of maps to scale; and for this purpose a 

 sheet of paper was prepared with geometrical squares, crossed by diagonal 

 lines. A collodion positive was taken of this, and projected, magnified eighty 

 times, on a screen covering one hundred square feet, and the image was 

 found, on accurate measurement, to be geometrically correct, the lines, etc., 

 being all free from curvature to the edge. 



FURTHER RESEARCHES BY M. NIEPCE ST. VICTOR OX THE ACTION 



OF LIGHT. 



Some time since, M. Niepce St. Victor announced the fundamental fact, 

 that a body exposed to solar radiation could act in the dark at a distance on 

 certain bodies, like light which emanated directly from the sun. The observa- 

 tions were made mostly with a cylinder of white pasteboard. M. Niepce has 

 since noticed that the pasteboard that has been exposed to the sun, and then 

 has been preserved, in the dark, in a cylinder of sheet tin (tinned iron), is 

 still active six months afterwards. This action of the chemical fluid calls to 

 mind radiant heat. Nitrate of uranium has, in a high degree, the property 

 of magnetizing the chemical fluid. On exposing to the sun, under a photo- 

 graphic proof, paper impregnated with nitrate of uranium, and then, at the 

 end of a quarter of an hour, plunging it into a solution of nitrate of silver, in. 



14 



