864 
PEOFESSOE W. A. MILLEE ON THE PHOTOG-EAPHIO 
elements of Gkove’s construction, a condenser being included in the primary circuit, 
whilst a small Leyden jar, exposing about 75 square inches of metallic coating upon 
each of its surfaces, was introduced into the secondary circuit. In this way a torrent of 
sparks could be maintained between the electrodes at e without any sensible variation of 
power, for ten minutes at a time, or longer if necessary. In these experiments, an ex- 
posure of the sensitive plate for five minutes in the camera was requisite. 
At a suitable distance behind the lens (about 26 inches*), a collodion plate coated with 
iodide, or occasionally with a rrrixture of iodide and bromide of silver, was supported in the 
camera, for the purpose of receiving the image of the spectrum f . The plate was excited 
by the use of a bath of nitrate of silver containing 30 grains of the rritrate to an oimce 
of water. The image was developed in the usual way by means of pyrogallic acid, in 
the proportion of one grain to the ounce of water, and fixed with cyanide of potassium. 
7. The spectra of electric sparks so obtained were remarkable for their great length ; 
indeed they extended beyond the termination of the visible rays for a space equal to five 
or six times the length of the luminous portion. 
For the convenience of comparing the results of the various experiments together, I 
have adopted an arbitrary fixed scale, the fiducial point of which is the line H in the 
solar spectrum. Calling this 100, the more refrangible rays are numbered onwards, and 
the less refrangible rays backwards from it, the line B in the solar spectrum being at 84 : 
the length of the spectrum from silver points extends from 96*5 upon this scale to I70'5. 
The solar spectrum for the purpose of this comparison was projected upon the collodion 
plate by means of a small mirror of polished steel (^, fig. 2) placed so as to form an angle 
of 45° with the surface of the plate carrying the slit, and to cover a portion of the vertical 
slit, as shown by an end view of the tube at fig. 3, whilst the direct image from the 
silver points fell simultaneously, parallel to that of the solar spectrum, upon the 
collodion plate in the camera. 
8. The following Table contains a list of the various substances subjected to experi- 
ment. All these bodies allowed the less refrangible rays to pass, but cut off the rays of 
medium and extreme refrangibility wherever absorption occurred at all. 
*■ This distance was found by experiment to give nearly a flat fleld, with the image of the slit formed by 
all the different rays in focus simultaneously. My friend and colleague, Professor J. C. Maxwell, kindly 
calculated for me the relative positions of lens and prism necessary to ensure an approximatively flat field for 
the visible rays. 
If the lens be placed between the slit and the prism, a very great difference occurs between the points oi 
convergence of the most refrangible and the least refrangible rays, amounting with the lens and prism which 
I used to nearly 14 inches. When the lens is before the prism, both coincide in augmenting the convergenc® 
of the more refrangible rays ; whereas when the lens is placed behind the prism, as shown in the figure, 
the convergence occasioned by the lens is neutralized by the prism, which now acts in the opposite direction 
upon the diverging rays as they fall upon it from the slit. 
t My friend Mr. Pizet, who assisted me in these experiments, prepared the collodion for me, following 
nearly the directions given by Haedwich in his ‘ Manual of Photographic Chemistry,’ 6th edition, p. 262. 
It was iodized with a mixture of equal parts of iodides of potassium and cadmium, and was perfectly uniform 
in its action, even for weeks after it had been iodized, if kept in the dark. 
