136 
In fig. I we have a lens mounted in a socket so as to turn 
horizontally, and behind it we have a piece of cardboard 
bent to form part of a cylinder, of which the focal length of 
the lens is the radius. 
Now, if in front of this we place a taper, an image of the 
taper is formed upon the cardboard screen. We may turn 
the lens backwards and forwards through a short arc, and yet 
you will see the image remains stationary. If we move the 
taper sideways keeping it at the same distance from the lens 
and turn the lens towards it, an image is produced on a 
different part of the screen. If a sufficient number of candles 
had been placed before the instrument and we had limited the 
action of the lens upon the screen to a narrow vertical band 
of light, the screen would have received consecutively the 
images of all the candles laid side by side, and perfectly 
distinct from each other. 
Now if for the cardboard we substitute a bent Daguerreotype 
plate, and if for the ordinary reading glass we put a good 
photographic lens, and for the series of tapers any landscapes 
or series of lighted objects, we shall have an exact reproduction 
of Martens’ camera. He took a number of beautiful views of 
Paris, from the bridges of the Seine, but the instrument 
failed in other hands than his. The difficulty of polishing 
and preparing large silver plates was of itself a serious 
obstacle, and a still greater want of success arose from the 
fact that Mr. Martens not being familiar with the practice of 
mechanics, did not attempt, or failed to produce the motion 
of the lens by self-acting means, and it requires the greatest 
skill and care to maintain a uniform motion for the length of 
time necessary for due exposure by means of the hand alone. 
Even in the hands of Mr. Mayall, so successful in large 
Daguerreotype plates, and so careful and intelligent an 
operator, the instrument failed completely. He worked for a 
long time at Niagara, unsuccessfully, before obtaining the 
few views he brought away with him. 
