The adoption of a reseau photographed upon the plate has greatly facilitated the procedure.
The cutter employed to rule these lines removes the silver in fine lines from the surface of the glass, Thus, if a photographic plate, before it is exposed in the telescope, is placed with its sensitive surface nearly in contact with the silvered surface of this reseau, and if parallel light, normal to the surface of the plate, is allowed to fall on the silvered film through the glass on which the film has been deposited, that light will pass through the fine lines in the silver film where the silver has been removed by the cutter, but will otherwise be intercepted by the silver film.
Thus a latent image of the " reseau-lines " will be formed on the sensitive plate, and, when the latter has been exposed to the sky in the telescope, we obtain, on development, a negative of the images both of the stars and of the reseau-lines.
It is practically impossible to work with the sensitive film in contact with the reseau-film, not only because dust particles and contact would injure the silver film, but also because the plate-glass used for the photographic plates is seldom a perfect plane.
For very refined work, however, the irregularities in the reproduction of the reseau may be studied by comparing the measures of the original reseau with the mean of corresponding measures of a number of photographed copies of it.
Reseau-square by means of a spider-line micrometer, a glass scale, on the plan shown in fig.
The image of the star is set updn the intersections of the lines of the central cross, and the positions of the reseau-lines are read off by estimation to - of a division on the glass scale.
The microscope or viewing telescope is fitted with a spider-line micrometer having two screws at right angles to each other, by means of which readings can be made first on one reseau-line, then on the star, and finally on the opposite reseau-line in both co-ordinates.
The image of a normal reseau-square, as viewed in the microscope, shall exactly coincide with the square formed by the fixed webs - that is to say, the image of the sides of a normal reseau-square shall measure exactly io screw-revolutions.
By means of the quick rack motions A and B move the plate so as to bring the reseau-square into the centre of the field of the micrometer; then, by means of the screw heads o, p, perfect the coincidence of the " fixed square " of webs, with the image of the reseau-square.