382 Mr. Towson om the proper Focus for the Daguerreotype. 



which has hitherto escaped observation. It appears from a 

 note appended to page 37 of the English translation of"Da- 

 guerre's description," that he does not use an achromatic lens ; 

 and from p. 62, that the focus he uses isobtained by advancing 

 or withdrawing the frame of the obscured glass until he ob- 

 tains the outlines of the subject with the greatest neatness. 

 This method would be most correct if the chemical rays were 

 identical with the luminous rays. If such were the case the 

 effect produced on his plate would be precisely that which 

 had appeared on his obscured glass. But it is a well-known 

 fact, that the chemical rays are more susceptible of refraction 

 than the luminous rays; it is therefore necessary, in order 

 to obtain the neatest effect, that the camera should be adjusted 

 to the focus of the chemical rays. 



M. Frauenhofer, by his investigation of the phaenomena of 

 the prismatic spectrum, has shown that the index of refraction 

 of each ray is as follows : 



I also find that the mean index of refraction of the invisible 

 chemical ray is for flint glass 1*693, and for crown glass 

 1 '5^6. The index for plate glass is also about the mean be- 

 tween those of flint and crown glass. 



When we adjust a camera to the point at which the figure 

 appears most distinct we obtain the mean focus of the lumi- 

 nous power of the united ray, because each coloured ray pos- 

 sesses a different degree of illuminating power; therefore the 

 appearance of the figure is mostly influenced by the yellow 

 ray, because it has the greatest degree of illuminating power ; 

 and least of all by the violet, because it yields the smallest 

 degree of light. 



The proportional light afforded by each ray is as follows : 

 Red -009, orange -048, yellow TOOO, green -440, blue '084, 

 indigo -010, and violet 'OOl. On the other hand, each ray 

 also tends to disturb the distinctness of the figure in propor- 

 tion to its distance from the mean focus of the pencil to 

 which it belongs; thus two rays would but occasion a similar 

 degree of indistinctness to that which one ray of equal power 

 would if situate at twice the distance from the mean focus of the 

 pencil to which it belongs. The elements of our calculation, 

 in ascertaining the point at which rays of various degrees 



