'l'HE APPARATUS.
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THE APP.AR.A TUS.
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The simple camera-obscura, invented by Porta in the year 1650, served for photographic experiments as long as the art remained in its infancy ; but as the art progressed, it was soon found that this instrument was too imperfect to satisfy the increasing demands made upon it. We have already seen in the Introduction, how a most material improvement was made in the camera by the combined exertions of Petzval and Voigtlii.nder. But even this improved camera is by no means everything that could be desired. The demands that may be justly made upon an apparatus truly deserving the epithet perfect, are as follows : it must give a large-sized picture, correctly drawn, and equally distinct in all parts; it must command a most intense light ; and it must be adapted equally for portraying, and for taking views of architectural objects. Now, up to the present time, the problem of fulfilling all these conditions equally, has not yet found its solution; indeed, one excludes the other. If the picture is to be large, of equal clearness and distinctness in all parts, even to the border, and equally so for near objects as for those farther off, the focal distance must be proportionately longer, and the aperture, through which the luminous rays enter, must be smaller; the necessary consequence of this mode of construction is, that the apparatus does not command a sufficiently intense light to answer the purpose of taking portraits with it. On the other hand, if the focal distance is lessened, and the aperture enlarged (with a lens suited to the purpose), the apparatus will indeed command a greater amount of light, and will accordingly be better adapted for the taking of portraits-but it will be found that this has been achieved only at the expense of the size of the picture, and of the desired correct delineation, and equal clearness and distinctness in all parts. An apparatus so constructed is, therefore, but imperfectly adapted for taking views of landscapes and of architectural objects ; and although small views may, if need be, be taken with it, by placing screens before it to reduce the aperture, the productions so obtained are very inferior to the views taken with an apparatus of greater focal distance and smaller aperture. This applies more especially to pictures