REPRODUCTION OF TRANSMISSION GRATINGS.
For laboratory work, cheap substitutes for the more expensive ruled gratings are easily obtained by placing a sensitised plate in contact with one of the glass trans mission type. The light cannot pass through the ruled parts, so that, On developing and fixing, a negative of the grating is obtained. This causes the inci dent light to diffract precisely as the original grating did. if mercury be placed at the back in contact with the film it will act as a reflection grating. For good definition, when used in this latter manner, the glass carrying the film must be of even thickness and have good plane surfaces. Talbot's Bands may be seen with a grating made in this way. Collodion plates are best. When examined under the micro.
scope, the deposited silver is seen to be in much smaller particles, and to be fairly evenly distributed. With ordinary gelatine plates, the silver occurs in larger, ir regularly sized aggregations. The glass
surface should be presented to the incident light coming from the collimator lens. This will avoid small refraction errors, and, when not in use, there should be a glass cover to protect the film, only ex posing the latter during the actual time of observation.
to be coloured, and which lies nearest to the collimator, is the one to be taken. The plane of Alie grating must now be making an angle of 45° with the incident rays of light. Round the circumference of the table which carries the prisms is marked a scale, and this enables the grat ing to be turned round through 45° in order to be at right angles to the incident light as shown. If it is a transmission grating, the telescope is moved back to position A, where a very bright yellow slit