916 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



plate than the lummous focus, which may prevent that nice adjust- 

 ment which is so desirable; besides, the question remains, whether 

 the lens can be made to form as distinct an outline as the reflector. 

 If I have not erred in my experiment with the lens and the 

 reflector, it would appear that the chemical and luminous rays do 

 not follow the same laws with respect to reflection and refraction. 

 Thus it is estimated that there is a loss of about one-half the light 

 from a speculum by reflection when the rays fall nearly perpen- 

 dicular to the surface. 



Call the rays falling on the speculum 100 



Loss one-half - 50 



50 

 Loss ^ by the plate being interposed 8 



42 



Lio-ht fallincr on a lens 100 



Loss by reflection from two surfaces . ,._ 16 



84 



Thus we have, after reflection, 42 parts, and after refraction 84 

 parts ; that is, half the area with the glass would be just as effect- 

 ual with light as the whole would with a reflector ; this would 

 give five inches as the diameter of the glass, and seven inches 

 that of the reflector, whereas by my experiment, 3| is the diam- 

 eter of the glass when it is equally efficacious with the chemical 

 rays as the reflector of seven inches. This would give nearly 

 twice as great a loss of the chemjcal rays, by reflection, when 

 compared with refraction, as there is of the luminous rays. If 

 there is this diflference between reflectors and refractors, there 

 should be a much greater allowance than three-eighths for the 

 loss from reflection from the flat speculum used with the lens. 

 This would bring the aperture of the glass about equal to that of 

 the reflector. I shall repeat the experiment more carefully when 

 I have time ; and if I am in error, will give j^ou the result. 



Yours, respectfully, 



A. S. WOLCOTT. 

 Jajies J. Mapes, Esq. 



Extract from the leading article in the "American Repertory," 

 New York, July, 1810, entitled " Remarks on the Daguerreotype, 



