482 M. A. Claudet on the Dynactinometer, an Instrument 



presently be explained ; but the second is more appropriate to 

 the object in view. 



The black disc may be made to revolve in snch a manner that 

 it shall cover a new segment of the large circle during each second, 

 or any other equal fraction of time. By that means the last seg- 

 ment will have received twenty times more light than the first, 

 and all the others in arithmetical progression. 



Having remarked, in the description I have given of the pho- 

 tographometer, that the difference of photogenic intensities are 

 hardly observable when they follow the arithmetical progression, 

 for which reason the instrument was so constructed that it indi- 

 cated the intensities in the geometrical progi'ession, I have for 

 the same reason adapted to the dynactinometer another circle 

 divided into eight segments. The first segment remains always 

 covered, in order to be represented black on the Daguerreotype 

 plate and mark the zero of intensity ; the second is exposed to 

 light during 1", the third during 2", the fourth during 4", the 

 fifth during 8", the sixth during 16", the seventh during 83", 

 and the eighth during 64". This series, which could be ex- 

 tended by dividing the circle into a greater number of segments, 

 is quite sufficient for all observations intended for practically 

 measuring the intensity of the photogenic light, and for com- 

 paring the power of object-glasses. 



The instrument is made to move by appljang the hand on a 

 handle fixed on the back at the extremity of the axis on which 

 the disc revolves. An operator accustomed to count seconds 

 by memory or by following a seconds' beater, can perform the ex- 

 periment with sufficient i-egularity ; but in order to render the 

 instrument more exact and moi'e complete, it can be made to 

 revolve by clock-work, which gives it at will either the arithme- 

 tical or the geometrical progression. This last movement pre- 

 sented some difficulty ; but I have been able to obtain it without 

 much complication in the machinery, and the apparat\is is within 

 the reach of the greater number of operators having estabhsh- 

 ments on a complete footing. 



For the instrument moving by hand, it is necessary that a 

 second person should open and shut the object-glass at a given 

 signal. But in adapting before the object-glass a flap connected 

 with a cord and pulley, the operator, holding the cord in the left- 

 hand, can open the flap at the moment that with the right-hand 

 he makes the disc revolve, and shut the apparatus when the revb- 

 lution is complete. 



When the instrument acts by clock-work, the object-glass may 

 be open and shut by the same means, at the signal given by a bell 

 which strikes at the commencement and at the end of the revo- 

 lution. 



