ISfe On a perucopk Camera Olscura 



mode similar to the precedino-, by adapting the multiple 



119 . ~ 



m = J _ ■ = — to the different density ot this glass. 



With respect to the construction of a micToscope on 

 periscopic principles, I believe the contrivance to be equally 

 new with the former, and equally advantageous. The 

 greal desidtrratuni in employing high magnifiers is suffi- 

 ciency of light; and it is accordingly expedient to make 

 the aperture of the little lens as large as is consistent with 

 distinct vision. But if the object to be viewed is of such 

 magnitude as to appear under an angle of several degrees 

 on each side of the centre, the requisite distinctness cannot 

 be given to the whole surface by a common lens, in con- 

 sequence of the confusion occasioned by oblique incidence 

 of the lateral rays, excepting by means of a very small 

 aperture, and proportionable diminution of light. 



In order to icmedy this inconvenience, I conceived that 

 the perforated metal, v^'hich limits the aperture of the 

 lens, might be placed with advantage in its centre ; and 

 accordingly I procured two plano-convex lenses ground to 

 the same radius, and applying their plane surfaces on op- 

 posite sides of the same aperture in a thin piece of metal 

 (as is represented by a section, fig, 4), I produced the de- 

 sired effect ; having virtually a double convex lens so con- 

 trived, that the passage of oblique pencils was at right an- 

 gles with its surfaces, as well as the central pencil. With 

 a lens so constructed, the perforation that appeared to give 

 the most perfect distinctness was about one- fifth part of 

 the focal length in diameter ; and when such an aperture 

 is well centred, the visible field is at least as much as 

 twenty degrees in diameter. It is true that a portion of 

 light is lost by doubling the number of surfaces; but this 

 is more than compensated by the greater aperture, which, 

 under these circumstances, is compatible with distinct 

 vision. 



Beside the foregoi.^g instances of the adaptation of peri- 

 scopic principles, I should not omit to notice their appli- 

 cation to the camera lucida ; as there is one wiriety in its 

 form, that was aol noticed in the description which I ori- 

 ginally gave of that instrument*. 



In drawing, by means of the camera lucida, distant ob- 

 jects are seen by rays twice reflected {d, fig. 5), at the same 

 time and in the same direction that rays (e) are received 



• Phil. Mag, ixviL p. S'iS, Nicholson's Journal, svii, p. 1. 



from 



