30 FERISCOPIC CAMERA AND MICROSCOPE. 



the line AR, parallel to AF, and equal to 19,8 divisions of the 

 primary lines j so that if r be = JO. then the line BC a= mr. 

 The distance AC being then divided into ten equal parts, 

 with their subdivisions, afforded the means of continuing the 

 same scale to any desired length. Since the first line BC was 

 laid down parallel to AF, and equal to mr, any other lines drawn 

 through corresponding numbers 7 and 7, 8 and 8, &c. will be 

 also parallel, and by preserving due proportion, will correctly 

 represent mr. Hence in all positions of the line FR, the same 

 similarity of triangles obtains, and the same proportion of F : 

 R : : mr : R — r -, and consequently the focal length, corre- 

 sponding to any assumed radii, is truly ascertained. 



For the purpose of duly proportioning the curvatures of 

 flint glass, a second line Ay might be laid down in a mode 

 similar to the preceding, by adapting the multiple m= 

 == — to the different density of this glass. 



Periscopic With respect to the construction of a microscope on peris- 



cope. 6 micros " copic principles, I believe the contrivance to be equally new 

 with the former, and equally advantageous. The great desU. 

 deratum in employing high magnifiers is sufficiency of light 3 

 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 consequence of the confusion occasioned 

 by oblique incidence of the lateral rays, excepting by means 

 of a very small aperture, and proportionable diminution of 

 light. 

 Two piano- In order to remedy this inconvenience, I conceived that the 



placedVace^o P er f° ratea< metal, which limits the aperture of the lens, might 

 face with a be placed with advantage in its centre -, and accordingly I 



central aper- p rocure( j two plano-convex lenses ground to the same radius, 

 ture in a plate r r & > 



between them, and applying their plane surfaces on opposite sides of the 



same aperture in a thin piece of metal (as is represented by a 



section, fig. 4), I produced the desired effect $ having virtually 



a double convex lens so contrived, that the passage of oblique 



pencils was at right angles with its surfaces, as well as the 



Dimension*, central pencil. With a lens so constructed, the perforation 



that 



