162 ANNUAL OF SCIENTIFIC DISCOVERY. 



reaction between the bodies in the tin tube, is the actual producing cause of 

 the effect," described by M. Niepce. 



PHOTOGRAPHS OF FLUORESCENT SUBSTANCES. 



At the British Association, Aberdeen, 18-59, "Dr. Gladstone stated, that it is 

 well known, on the one hand, that the chemical action of light resides 

 mainly in the most refrangible rays, and on the other hand that these rays 

 are altered in their refrangibility and effect on the visual organs by fluores- 

 cent substances. It occurred to the author that such substances would prob- 

 ably exert little photographic action. Hence he had made two drawings on 

 sheets of white paper, one in an acid salt of quinine, the other in a very pale 

 solution of chlorophyll, and had taken photogi-aphs of them. Although the 

 drawing in quinine was quite undistinguishable from the white paper, and 

 the chlorophyll drawing nearly so, when they were viewed in the camera, for 

 adjusting the focus, they were strongly marked on the photographic image by 

 the little chemical action that had been exerted by them. The sheets of 

 paper, and the drawings developed on the glass plate, were exhibited, show- 

 ing that what theory had suggested as probable, was true in fact. 



TIME AND PHOTOGRAPHY. 



We have heard it affirmed that a fly is a medium-sized object in the scale 

 of living beings, meaning that there ai*e objects as much smaller than a 

 fly, as an elephant or whale are larger, and this we believe to be true. But 

 what shall we say to a second in respect to photographic action? Taking six 

 hours as the maximum time of exposure, we can show differences in times 

 of exposure and variations in active action on the other side of a second of 

 time, far exceeding anything ever dreamed of in the ordinary practice of 

 photography. In taking photographs of rapidly moving objects, the 

 waves of the sea, for example, we have been obliged to judge of the proper 

 exposure requisite to bring out the half-tints, and estimate differences of 

 time varying between the l-SOth and the 1 -120th of a second. Expressions 

 like these are, however, enormous when compared with the time occupied in 

 other photographic experiments. Thus in solar photography, according to 

 the experiments of Mr. \Vaterhouse, an image was produced in a space of 

 time not longer than the l-OOOOth of a second, even when a slow photo- 

 graphic process was used; and when wet collodion was employed, one-third 

 of the above time only was requisite, or l-27000th part of a second. This 

 duration, however, inconceivably short as it appears, will seem to be a toler- 

 able length of time, when we try to bring the mind to appreciate the rapidity 

 with which Mr. Talbot performed his crucial experiment at the Royal Insti- 

 tution, when he photographed a rapidly revolving wheel, illumined with a 

 single discharge of an electric battery. To a casual observer, or reader of 

 this experiment, the wonderful part appears to be, that the wheel appeared 

 perfectly well defined and stationary in the photograph, although in reality 

 it was being rotated with as great a velocity as multiplying Avheels could 

 communicate to it. A little further consideration will, however, show that 

 the time occupied in the revolution of the wheel was a planetary cycle com- 

 pared with the duration of the illuminating spark, which, according to the 

 most beautiful and trustworthy experiments of Wheatstone, only occupies 

 the millionth part of a second. Photographic Neics. 





