220 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV. 



but simple mechanism, at such a distance from the back- 

 ground, or so arranged with resppct to the camera, that 

 his shadow shall not be copied as a part of his body ; 

 the aperture of the camera should be three and a half or 

 four inches at least, indeed the larger the better, if the 

 objective be aplanatic. 



If two mirrors be made use of, the time actually occu- 

 pied by the camera operation varies from forty seconds to 

 two minutes, according to the intensity of the light. If 

 only one mirror is employed, the time is about one fourth 

 shorter. In the direct sunshine, and out in the open air, 

 the time varies upwards from half a minute. 



Looking-glasses which are used to direct the solar 

 rays after a short time undergo a serious deterioration, 

 the silvering assuming a dull granular aspect, and losing 

 its black brilliancy. Hence the time, in copying, becomes 

 gradually prolonged. 



The arrangement of the camera above indicated gives 

 reversed pictures, the right and left sides changing places. 

 Mr. Woolcott, an ingenious mechanician of this city, has 

 taken out a patent for the use of an elliptical mirror for 

 portraiture; it is about seven inches in aperture, and 

 allows him to work conveniently with plates two inches 

 square. The concave mirror possesses this capital advan- 

 tage over the convex lens, that the proof is given in its 

 right position, that is to say, not reversed; but it has the 

 serious inconveniences of limiting the size of the plate, 

 and representing parts that are at all distant from the 

 centre in a very confused manner. With the lens, plates 

 might be worked a foot square, or even larger. 



Miniatures procured in the manner here laid down are 

 in most cases striking likenesses, though not in all. They 

 give, of course, all the individual peculiarities a mole, a 

 freckle, a wart. Owing to the circumstance that yellow 

 and yellowish browns are long before they impress the 



