OP ARTS AND SCIENCES. 



187 



front surface, any light reflected from it is thrown entirely away from 

 the photographic plate. B is the objective, wliicl) is corrected for the 

 chemical, and not for the visual rays. The distance between the 

 objective and mirror is made as small as possible, consistently with 

 keeping the latter clear of the shadow of the former. D is the reti- 

 cule, the construction and use of which will be ex[)lained presently. 

 C is the photograpliic plate, the sensitive surface of wliich faces the 

 objective. Tlie rays from the sun S are reflected by tlie mirror A 

 through the objective B, and after traversing the reticule D they form 

 an image upon the photographic plate C. 



The reticule consists of a system of squares, formed by the inter- 

 section of two systems of very fine, straight lines, wbicli are drawn 



upon one side of, and respectively paral- 

 lel to the edges of, a thin, square plate 

 of piano-parallel glass ; as shown in 

 Fig. 2. In each of these linear sys- 

 tems the number of lines is odd, and 

 the middle line is drawn through the 

 centre of the plate. This reticule is 

 fixed at D, Fig. 1, with its rided sur- 

 face toward, parallel to, and two or three 

 millimeters distant from, the sensitive 

 surface of the plate C. Moreover, one 

 of the two systems of lines is set as 

 nearly as possible vertical, and its inclination is accurately deter- 

 mined ; and as an additional safeguard, a plumb line, consisting 

 of a silver wire about 0.05 of a millimeter in diameter, is suspended 

 between the reticule and the photographic plate, in suih a position 

 that it may hang freely, and at the same time be very nearly in the 

 vertical plane passing through the centres of the reticule and objective. 

 As the light from the objective traverses the reticule bifore it reaches 

 the photographic plate, the shadow, both of the reticule and of the 

 plumb line, is impressed upon every picture taken with the apparatus ; 

 and thus three different ends are gained : Firstly, by comparing the 

 squares of the reticule witli the corresponding ones upon the picture, 

 every thing relating to the shrinkage of the c'dlddion can be deter- 

 mined ; secondly, the impression of the plumb-line, and also that of 

 the vertical lines, furnishes upon each picture a fixed direction from 

 which to measure angles of position ; and, thirdly, the intersection 

 of the middle vertical with the middle horizontal line furnishes a 

 fixed point, which will hereafter be designated as the centre of the 



_.pi^. 



Fig. 2. 



