34 Photographic Notes. 
the “standard candle” as a reference unit, are based on this. 
Count Rumford’s method was shown with electric light against 
gas. It depended on comparing the shadows of a rod cast from 
two sources of light, the distance of each source from the rod 
being measured in inches. In this case the small incandescent 
light at 130 inches gave an equal shadow to that of gas at 105 
inches. When two lights are much out of proportion, it is diffi- 
cult to measure their relation—as, for example, the sun and the 
moon ; but the most reliable results show that the light of the sun 
is some 250 thousand times greater than that of the moon. Our 
eyes so gradually get accommodated to the change from sunlight, 
through twilight to dark, and to moonshine, that this enormous 
difference is far greater than most people would guess. In allud- 
ing to and illustrating other methods of photometry, mention was 
made of the work of Fox Talbot, which, the lecturer said, was 
overlooked too much, because his papers were concise and short, 
and therefore had not that appearance of learning which long 
papers nobody read usually conveyed. In the illustrations of the 
laws of reflection the sharp definition given to the electric lantern 
was observable in a very marked manner. It was mentioned that 
the silver mirrors in astronomical telescopes now used reflect 95 
per cent. of the light, polished steel giving about 50. 
Photographic Notes. 
Washing Prints with the aid of Iodine.—The Paris A/oniteur 
states that Mr. Oakley is accustomed to use iodine to get rid of 
the last traces of hyposulphite in paper prints. His manner of 
procedure is very simple :—To the washing water he adds a small 
quantity of the tincture of iodine, which gives to the back of the 
print a slight bluish tint ; he then continues the washing until the 
tint has disappeared. We know that in the presence of hyposul- 
phite of soda, the blue tint (which is due to the action of iodine 
on the starch in the paper) cannot be produced. 
Photographers’ Paste.— Water, goo parts ; glycerine, 100 parts ; 
starch, roo parts. Dissolve with the aid of heat. Cardboard 
does not cockle when this paste is used. 
Instantaneous Magnesium Light.—It is said that an instan- 
taneous magnesium light may be obtained by mixing 12 parts of 
chlorate of potash, 6 parts of magnesium in powder, and 1 part of 
pirocyanuret of potassium. On applying a light, this mixture 
gives a sudden flame, very brilliant, which lasts from one-thirtieth 
to one-fiftieth of a second where small quantities are used, and it 
is asserted that the light produced is more intense than that of 
