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II. On the Stereoscopic Relief in the Chimenti Pictures* 

 By Sir David Brewster, K.H., F.R.S. 



To William Francis, Esq., Ph.D. 



Dear Sir, 



MY attention has just been called to the following statement 

 by Professor Edwin Emerson, of Troy University, U.S., 

 published in the Philosophical Magazine for February 1863. 



u Ho prove this [that the stereoscopic qualities of the Chimenti 

 pictures are ' evidently accidental'], let anyone execute a pen- 

 and-ink sketch, and then let him make as perfect a copy of it 

 as he can without careful measurements : now place these two 

 drawings in the stereoscope, and you get the same kind of effect 

 seen in the Chimenti drawings, and for the same reason^ 



This appeal to any one for a scientific truth is rather an un- 

 usual mode of ascertaining it. Has Professor Emerson himself ex- 

 ecuted and copied any such sketch, and obtained the result which 

 he assumes ; and if he has, why has he not distinctly published 

 the fact, and stated the nature of the sketch, and the precise 

 kind of relief which he obtained. Had he done this, we might 

 have challenged the accuracy of his experiment, and suggested 

 a better mode of arriving at the truth. 



In the absence of this information, however, we beg to ask 

 him if he made a correct copy of one or other of the Chimenti 

 drawings ; and if he did, why he did not advise any one to do 

 the same in order to obtain the only satisfactory proof of his 

 assertion, that the relief in the Chimenti pictures is accidental. 



Now we beg to tell Professor Emerson that we took this very 

 method, and the only judicious one, of arriving at the truth. We 

 went to one of the masters of the School of Art under the Board 

 of Trustees in Edinburgh, with whom we were not personally 



very slight variation of atmospheric pressure at the conclusion of an expe- 

 riment extending over a long period of time would vitiate any deduction 

 relating to a point of such extreme delicacy, may we not legitimately ques- 

 tion whether all the precautions necessary for securing a reliable result were 

 observed in this case ? Moreover Regnault's recent experiments in relation 

 to the " Elastic Forces of Vapours in vacuo and in Gases," while they con- 

 firm Daltou's law as a theoretical law, do not appear to verify it as an abso- 

 lute physical fact — probably from the circumstance that it is impossible to 

 find any two vapours or gases entirely deprived of all reciprocal action. 

 Again, the researches of Bunsen (' Gasometry/ translated by Roscoe, 

 London, 1857, p. 216) seem to show that the diffusive interchange between 

 gases " does not occur in the relation of the inverse square roots of the 

 specific gravities," and consequently that Graham's law is only approxi- 

 mately correct. 



Phil. Mag. S. 4. Vol. 27. No. 179. Jan. 1864. D 



