458 Mr. Claudet on a Focus Equalizer for Photography. [May 2, 



questions of optical photography, to calculate the result of the gradual 

 increase and reduction of the power of the double combination, in con- 

 junction with the alteration of focus. Dr. Sommer entered thoroughly 

 into the subject, and soon sent me a series of elaborate formulae, showing 

 that, although for all practical purposes in photography the movement of 

 one of the two lenses, as I had proposed, fulfilled the object I had in view, 

 still he found that a more scientific consideration of the subject called 

 for a modification in my plan ; which was that, instead of moving only one 

 of the lenses, the same degree of their separation should be imparted by 

 moving the two lenses in contrary directions from the fixed centre of the 

 combination, and in different proportions, according to the distance of the 

 object. These differential proportions were indicated in a table calculated 

 by Dr. Sommer which he sent me. 



This presented another difficult and unexpected problem, the solution 

 of which was indeed most perplexing. But I did not like that it should be 

 said that my plan was not completely in accordance with the mathematical 

 laws of optics ; and I set to w^ork at finding a mechanical means by which I 

 could avail myself of the scientific calculations of Dr. Sommer. 



I have found such means ; and it turns out indeed that by my mechanical 

 construction the differential movement can be effected, not only as readily 

 and easily, but with a greater command and steadiness than by moving 

 only one lens. The following is a description of the arrangement : — 



Description of the Focus-Equalizer. 



The tube, containing at each end the lenses A and B, is divided into 

 two parts, sliding in the principal tube S S S S fixed in the front of the 

 camera at V V. 



Each tube has a strong pin, L and L'. These two pins are intended to push 

 the tubes to and fro from the centre of the combination on the line P P' 

 by means of the mechanical piece N N' N" in the shape of a sextant, 

 having two slits, M M and M' M', cut at an angle of 36°. Now the sextant, 

 being mounted on a sliding bar q q, fixed in a socket holding to the tube 

 S S S S at P P', can be made to move to and fro on the line P P' by means 

 of a rack and pinion moved by a handle V on the axis R. While the 

 sextant moves in the line P P', the two slits will act on the two pins, and 

 gradually increase the separation of the tubes ; and on making the sextant 

 move back from P' to P, the sHts will bring the two pins nearer each other, 

 and decrease the separation of the tubes. • 



It will thus be easily understood how we can increase and reduce the 

 separation of the two lenses from the centre of the combination ; but 

 we have now to explain how we can produce the differential movement 

 according to the mathematical formulae calculated by Dr. Sommer. 



The arc of the sextant is divided into 100 parts, in two rows one against 

 the other. The divisions on the outer limb have their zero on the left, 



