Mr. Towson on the proper Focus for the Daguerreotype. 38S 



of refrangibility produce the most distinct effect, must 

 therefore consist both of the ilhmiinating power of each portion 

 of the spectrum, and its distance from the point required. 

 By a calculation founded on these data, we find that the 

 figure appears most distinct at the focus of the central yellow 

 ray. 



It must however be evident that this focus ought not to be 

 used for photographic purposes, since the yellow ray, although 

 it yields the greatest light, produces but a slight degree of 

 chemical action, whilst the chemical effect of the violet ray is 

 greater than that of any other luminous ray, but its illuminating 

 power is the least ; the rays that produce even a greater che- 

 mical action than all the luminous rays combined possess 

 no illuminating power. It has also been shown by Dr. Her- 

 schel that the extreme red ray and the invisible ray beyond 

 the red portion of the spectrum produce a chemical effect of 

 a contrary nature to that of the other rays. These considera- 

 tions are sufficient to convince us not only that the chemical 

 focus is differently distant from a lens than its luminous focal 

 Jength, but also to prove that the distance between the two 

 foci is sufficiently great to produce considerable practical re- 

 sults. It therefore becomes an investigation of considerable 

 importance as connected with the photographic art, to ascer- 

 tain the situation of the mean chemical focus of a lens. In 

 conjunction with the data our previous observations have af- 

 forded, the elements of such a calculation must consist of the 

 chemical power of those portions of the spectrum as have 

 not already been noticed, which is as follows. Taking 

 that of the invisible chemical ray as unity, that of the green 

 will be -01, • the blue •!, the indigo -3, and the violet '45. 

 With these data, and adopting the same formula we used in 

 calculating the mean luminous focus, we discover the mean 

 chemical influence to be without the limits of the luminous 

 portion of the spectrum, very near the extreme violet ray, and 

 that for all practical purposes we may find this focus for any 

 lens by multiplying its distance from the point at which the 

 figure appears most distinct by the factor "969 if it be of flint 

 glass, '976 of plate glass, or '984 of crown glass. Thus the 

 chemical focus of a lens whose luminous focus is 16 inches 

 would be if composed of flint glass about 1 5*504, of plate glass 

 15"616, or of crown glass 15''744 inches*. To demonstrate 



* From this result we might imagine that crown glass would be the 

 best material for photographic lenses. This however is not the case. The 

 least dispersive lenses intercept the greatest number of chemical rays, 

 and therefore those of crown glass, and consequently achromatic lenses, 

 cannot be advantageously employed for photographic purposes. This ob- 



