1867.] Mr. Claudet on a Focus Equalizer for Photogimphy. 459 



and the 100 division on the right ; on the inside limb the divisions are in 

 a contrary direction. 



By means of the endless screw X acting on the toothed edge of the 

 sextant, it can be moved on its horizontal axis, so that any of its divisions 

 may be brought under the index fixed on the middle bar q q. 



Now, supposing that by the table of Dr. Sommer the lens A for a 

 certain distance of the object should move 0*235, and the lens B 0*765 of 

 the whole space by which the lenses require to be separated or approxi- 

 mated, we turn the endless screw until the index is on the 23^ division of 

 the inside scale, and of course on the 76^ division of the outside scale. 



In that position of the sextant the slits M M and M' M', by means of the 

 pins attached to the tubes of the lens A and of the lens B, will make them 

 accordingly move — A in the proportion of 0-235, and Bin the proportion 

 of 0*765 of the whole space. 



If for another distance the lens A should have to move 0*333, and the 

 lens B 0*666, setting both limbs of the sextant to these divisions, the lens 

 A will move 5, and the lens B § of the whole space. 



If we wanted to move the two lenses in the same proportion, the sextant 

 should be set so that the 50th division of both scales should be under the 

 index. 



And, finally, if, for the sake of comparative experiments, it were wanted 

 to move only the lens A or the lens B, the slit of the pin for either 

 and the zero of the scale should be placed under the index, by which that 

 lens would be completely stationary, and the whole motion imparted to 

 the other. 



