Electrolysis of Secondary Compounds. 1 75 



able, the aperture admits of contraction ; and, by reducing it 

 to one-tenth of the compound focus, an image may be obtained 

 of exquisite fineness. 



Two lenses of 24 inches focus (3 inches diameter), placed 7 

 inches apart, have the same conjugate foci as a single lens of 

 13 inches; and, with an aperture of 1*3 inch, the picture is 

 beautifully defined : over 15° it is of a very high and uniform 

 quality under a magnifier; and within 20°, there is little differ- 

 ence perceptible to the naked eye. 



This combination is in strict accordance with the periscopic 

 principle of Dr. Wollaston ; and also, with that of the Cod- 

 dington lens ; I am not awjire that it has yet been applied to 

 the camera obscura, but it seems to be not less applicable to 

 that instrument than to microscopes. 



It might, perhaps, be improved by filling the space between 

 the lenses with a blue coloured medium, of the same refrac- 

 tion as the glass; the focus would be shortened and the pho- 

 tographic power increased ; and the photographic rays asso- 

 ciated with the red and yellow end of the spectrum being 

 absorbed, would cease to interfere with the unity of the focus. 



I avail myself of this opportunity to correct a mistake in an 

 article on " the Calotype " in the Philosophical Magazine of 

 May 1844. At p. 326, § 17, line second from the foot of the 

 page, for "a pint of water" read halfo. pint (ten ounces). 



London, July 15, 1844. 



XXVII. Additional Researches on the Electrolysis of Secondary 

 Compounds. By J. Frederic Daniell, Esq., D.C.L., 

 For. Sec. U.S., Prof, of Chem. in King^s College, London, 

 and W. A. Miller, Esq., M.D., Dem. of Chem. in King's 

 College, London^. 



n|^HE authors of the following paper having agreed to work 

 -■■ together upon the subject of the electrolysis of secondary 

 compounds as opened by one of them, in two letters addressed 

 to Dr. Faraday, and honoured with a place in the Philosophi- 

 cal Transactions for 1839 and 1840f, have arrived at some 

 results, which probably will not be without interest to the 

 Royal Society, and which they have now the honour to com- 

 municate. 



In the two papers just alluded to, the following points were 



* From the Philosophical Transactions for 1844; having been received 

 by the Royal Society February 15, — read February 25, 1844. 



[f An abstract of these papers was given in Phil. Mag. S. 3. vol. xvii. 

 p. 349.— Edit.] 



