OF ARTS AND SCIENCES. 485 



that part of the exposure which is with the full aperture. To insure 

 this result, if a slot is made to pass between the lenses, it should be as 

 long as possible in proportion to the diameter of the aperture. The 

 ends of the slot usually have one of the three following forms. 

 They are either semicircular and concave, rectangular, or semicircular 

 and convex.* Sometimes a single slot is used, and sometimes two, 

 which slide past one another in opposite directions. "We thus have six 

 methods of opening and closing the aperture, all in common use. Now 

 it is quite evident that all cannot be right, and perhaps the simplest 

 way of studying them will be by the following diagrams. (See page 

 486.) 



At the top of the page are shown the three terminations. The 

 vertical lines underneath show the distances which the shutter would 

 move while four points of the circular aperture between the lenses are 

 being exposed. The points selected are the centre, c, the upper ex- 

 tremity of the vertical diameter, t, the lower extremity, b, and the end 

 of the horizontal diameter, s. The shutters are supposed to full verti- 

 cally and uniformly, and the first series represents the case where the 

 central vertical dimension of the aperture in the shutter equals the 

 diameter of the aperture between the lenses. If the circular hole 

 passes the lens, the centre of the aperture between the lenses is 

 exposed, while the shutter moves through the diameter of the hole. 

 The top of the aperture is exposed for the same length of time, but its 

 exposure is half over before that at the centre begins. The exposure 

 at the bottom does not begin until that at the top is over. The ex- 

 posure at the side is merely for an instant, and therefore does not 

 count for anything. If the hole in the shutter were made somewhat 

 larger than that between the lenses better results would be obtained, 

 and the ex2iosures would resemble more nearly those represented in 

 the second figure, with a square aperture. If the shutter were placed 

 in front of the lens, it will now be seen that the bottom of the plate 

 would be photographed before the top, and that distortion would be 

 thereby produced. By placing it between the lenses, however, no 

 trouble of the sort can occur, as every portion of the aperture exposes 

 all parts of the plate at once. With a square slot all portions of the 

 aperture between the lenses receive an equal exposure. With the 

 third form of slot the sides receive double the exposure of the top, 

 bottom, or centre. 



* A diamond-shaped slot is also sometimes used; but this may be considered 

 under the first class. 



