484 PROCEEDINGS OF THE AMERICAN ACADEMY 



objection. If the exposer is of a form opening and closing from the 

 centre, the central portion of the plate will be longer exposed than the 

 rest, thereby producing a " flare spot," as it is called. But if one is to 

 make a specialty of instantaneous pictures, one lens may be devoted to 

 that work, and a lens-tube constructed for the purpose. The attach- 

 ment of an exposer properly constructed will not interfere with long 

 exposures. If placed between the lenses, the shutter will be nearer 

 the point of support for the camera, and consequently the jar caused 

 by the exposure will be less. All portions of the plate are exposed at 

 the same instant, and for an e(iual length of time ; lience there is no 

 flare spot produced from this cause. From the above it will be seen 

 that the place presenting the most advantages for the shutter is be- 

 tween the lenses. 



(2.) Construction of the Exposer. — The exposure is made either 

 by raising a flap, or by causing a single or double slot to pass by the 

 lens. The object of raising a flap is to expose the top of the picture 

 less than the bottom; but sometimes one does not want to expose the 

 top less than the bottom, and there is no more reason for doing so in 

 instantaneous work than with long exposures. Moreover, there are 

 other contrivances for doing this same thing in other ways, as by 

 holding a board in front of the upper part of the lens, etc. In any 

 case it retards the whole exposure more or less. A flap can never 

 be made to work as quickly as a sliding slot, therefore it cannot be 

 used for very rapid exposures ; and, moreover, it cannot be placed be- 

 tween the lenses, so that for the ideal shutter it is ruled out. Some- 

 times a slot is made to open the lens to its full aperture, to stop, and 

 then return the way that it came. If it comes up from below, and is 

 placed in front of the lens, it will evidently give the foreground a 

 longer exposure than the sky, but it cannot be made to work rapidly, 

 and the jar caused by its reversal comes at just the wor?t possible 

 time, — in the middle of the exposure. The arrangement, therefore, 

 emi3loying the continuously sliding slot, seems to be the best one. 



(3.) Shape of the Aperture in the Exposer. — The exposure of any 

 shutter may be divided into three parts : that while the shutter is open- 

 ing, that while it is opened to its full extent, and that while it is closing. 

 Now any bright moving object will begin to form an image soon after 

 tlie shutter begins to open, and will continue to form one until it is 

 nearly closed. A dark object, on the other hand, will only produce an 

 image during the exposure by the full aperture. But the exposure must 

 be prolonged until the dark object is fully taken ; therefore the opening 

 and closing of the shutter must be as rapid as possible in proportion to 



