216 Mr. A. Claudet on the Optics of Photography. 



My -focimeter, a model of which is on the table, is made of 

 eight separate segments of a disk, mounted spirally on a hori- 

 zontal axis of 12 inches, corresponding with the optical axis of the 

 lens ; the segments are all separated and distant about 1 \ inch 

 from each other. In a front view they form on the ground 

 glass the image of a complete and regular disk. The segments 

 are covered with some uniform and well-defined devices ; and the 

 centre of each is marked with its number, from 1 to 8. The 

 first segment is the nearest, and the last the furthest from the lens. 



By moving slightly and slowly forward and backward the fo- 

 cusing or ground glass, any one of these segments, and all in 

 succession, may be brought into focus. If we focus upon No. 4, 

 for example, we see that the segments before and behind gra- 

 dually lose their sharpness, in a greater or less degree, according 

 to the quality of the lens ; and from that experiment we may 

 judge of what is empirically called the depth of focus of the lens. 

 By comparing at the same time the photographic image with the 

 image we had on the ground glass, we see if the visual and che- 

 mical foci agree, or to what extent they differ. But our present 

 object not being to test whether the chemical and visual foci 

 agree, we will take a lens in which we know that they coincide. 



Now, supposing that we focus upon No. 1, we shall find that 

 the photographic image of that segment will be very well defined, 

 No. 2 a little less, No. 3 and all the others until No. 8 gradu- 

 ally losing their sharpness, so that No. 8 will be the most indis- 

 tinct. In the same way, if we take a portrait so that the nose is 

 on the plane of No. 1, this part of the face will be well defined ; 

 the eyes, which are on the plane of No. 2, will be a little less 

 well defined ; the ear, on the plane of No. 3, still less defined ; 

 and if the body is obliquely turned, the shoulder, which corre- 

 sponds with the plane of No. 8, will be considerably confused. 



Experimenting again upon the focimeter, let us suppose that, 

 after having operated with No. 1 in focus, we move the frame 

 holding the plate to a point previously marked on the camera- 

 board where No. 8 is in perfect focus. If we then expose the 

 plate a second time, or rather continue the exposure, we shall 

 find that upon the first confused image of No. 8 a new image 

 well defined has been impressed, and at the same time a confused 

 image of No. 1 will have been impressed upon the first image of 

 No. 1 which was well defined. 



In examining the result, we shall find it better than if the 

 second impression on both segments, No. 1 and No. 8, had not 

 been taken. In the middle of a confused image of No. 1 and 

 No. 8 we shall have one perfectly defined, the whole having the 

 appearance of the shadow of a pin not quite in contact with the 

 surface ; that shadow being slightly blended from dark to light, 



