Problem of Photographic Reproduction. 739 



Another method, simpler in execution, consists in super- 

 posing a considerable number (n) of similar pictures. In 

 this way the contrast is multiplied n times. Kays from a 

 small, but powerful, source ot light fall first upon a colli- 

 ihating lens, so as to traverse the pile of pictures as a parallel 

 beam. Another condensing lens brings the rays to a focus, 

 at which point the eye is placed. Some trials on this plan 

 made a year ago gave promising results. Ten lantern-slides 

 were prepared from a portrait negative. The exposure 

 (to gas-light) was for about 3 seconds through the negative 

 and for 30 seconds bare, i. e. with negative removed, and the 

 development was rather light. On single plates the picture 

 was but just visible. Some rough photometry indicated that 

 each plate transmitted about one-third of the incident light. 

 In carrying out the exposures suitable stops, cemented to the 

 negative, must be provided to guide the lantern-plates into 

 position, and thus to ensure their subsequent exact super- 

 position by simple mechanical means. 



When only a few plates are combined, the light of a 

 TVelsbach mantle suffices ; but, as was to be expected, the 

 utilization of the whole number (ten) requires a more 

 powerful source. Good results were obtained with a lime- 

 light ; the portrait, barely visible at all on the single plates, 

 came out fairly well under this illumination. If it were 

 proposed to push the experiment much further by the combi- 

 nation of a larger number of plates, it would probably be 

 advantageous to immerse them in benzole contained in a 

 tank, so as to obviate the numerous reflexions at the surfaces. 



It has been mentioned that in the above experiment the 

 development of the plates was rather light. The question may 

 be raised whether further development, or intensification, 

 might not make one plate as good as two or three superposed. 

 I think that to a certain extent this is so. When in a recent 

 experiment one of the plates above described was intensified 

 with mercuric chloride followed by ferrous oxalate, the 

 picture was certainly more apparent than before, when 

 backed by a sufficiently strong light. And the process of 

 intensification may be repeated. But there is another point 

 to be considered. In the illustrative experiment it was con- 

 venient to copy all the plates from the same negative. But 

 this procedure would not be the proper one in an attempt 

 to render visible the solar corona. For this purpose a good 

 many independent pictures should be combined, so as to 

 eliminate slight photographic defects. As in many physical 

 measurements, when it is desired to enhance the delicacy, 



