19 REPORT—1850. 
as M. Nervander’s, and thrice as good as Cassini’s, when the variations from day 
to day are considered. When however we consider the total difference of the ex- 
treme positions of the weight-indexes during the whole period, the ratios for the 
same torsion force are— ͵ 
Cassini = 26 ; Nervander= 14; Brown= 18; 
in which case M. Nervander’s construction has the advantage ; it is my belief how- 
ever that this advantage would not continue, and that when the threads have been 
suspended for a longer period my own thread will show also less amplitude of the total 
variation. I should remark, that I believe the conditions in the preparation of the 
threads were as nearly as possible equal. Mr. Hogg had never made a thread 
before according to either construction, and he removed the torsion from the fibre 
for each of the threads, which was taken from the same reel. 
Photography.—On a New Instrument called the Dynactinometer for com- 
paring the Power of Object- Glasses, and for measuring the Intensity of the 
Photogenic Light. By M. Ciauvet. 
The author announced several years ago (in 1844) that in achromatic lenses the 
photogenic focus did not coincide with the visual focus. Until that discovery no 
photographer was certain of obtaining a well-defined picture, because the image pro- 
duced on the ground glass was no guide for the correctness of the photogenic image 
which was at a different focus. Soon after M. Lerebours of Paris investigated the 
subject, explained the cause of the difference, and indicated the means to avoid it. 
Since that time opticians have endeavoured to construct lenses in which the two foci 
agree, but M. Claudet proves that it is impossible to construct lenses in which the 
two foci generally agree for all the distances of objects, and with all the modifications 
and the quality of light. He has lately discovered that there is a continued varia- 
tion between these two foci, and he enumerates a series of experiments which prove 
the truth of this fact and render necessary new formule for photogenic optical com- 
binations. The author observes that a good telescope may make a very bad camera 
obscura, and a good camera a very bad telescope. He has by many experiments 
proved that the lenses the most active in the photogenic operation are those in which 
the two foci are the most separated, and for that reason he prefers these last. 
In order to elucidate this phenomenon and generally to compare the power of all 
kind of lenses, he has contrived an instrument which he calls Dynactinometer, and 
which fulfills this double object. The instrument is composed of two discs, one 
black and the other white, each having a slit so arranged that the black disc can by 
a gradual superposition cover the whole white disc ; this last is marked with divisions 
like a watch-dial. In placing two cameras supplied with two different lenses before 
the dynactinometer, and making it revolve gradually by the hand, two Daguerreotype 
plates, placed one in each camera, receive at the same moment the image of the dial ; 
and as the black disc stops the effect on the segment as long as it covers it, it is 
evident that a greater power in ore lens will show each corresponding segment more 
intense than the other. These segments being numbered, it is easy, in comparing 
the two segments on each plate, having the same intensity, to judge by the number 
the proportion of the effect. This is the principle of the dynactinometer; by it M. 
Claudet has been able to observe that two spaces of the same area, taken one in the 
centre and the other near the circumference of the lens, although giving the same 
intensity of light, do not produce the same intensity of photogenic action, and also 
that lenses do not always present the same comparative power. From these facts 
he argues, that when the yellow rays are more or less abundant, they, by their known 
antagonistic action, interfere more or less with the photogenic effect, and that they 
destroy it when they are in certain proportions. This enabled him to offer an hy- 
pothesis of the cause of the variations between the two foci. The central parts of 
the surface of a lens concentrate with the photogenic rays more yellow rays than the 
other parts, and when these yellow rays are in excess they neutralize the action of the 
photogenic rays. In this case the centre does not operate photogenically, although 
it contributes to the visual image ; the central photogenic rays being less refracted 
