CONDENSER. The condenser is a thick convex lens, of moderate focus, uncorrected either for spherical or chromatic aberration, and used for the purpose of directing light through any transparent object intended to be copied, either in the copying or microscopic camera, in the manner described in the article on copying. Its use and mode of action will be understood from the following diagram. (Fig. 12.) A B is a transparent object to be copied, / m the diaphragm of a lens, or the front lens of a combination, 0 an artificial light (the oxy-calcium light, for instance), D E the condenser.
The condenser is larger than the object to be copied ; its principal focal length should be a little shorter than its distance from the incan descent lime ball 0 ; and the image of 0 should be formed very indistinctly, and with great spherical aberration in the neighbour hood of / m. All the rays included within the angle D 0 E will then be transmitted through A B and the copying lens, and the images of the transparent parts of A B will be formed by diverging pencils, as shown at the points A, 'C, B, and in the manner described in the article on copying,. (See " Copying.") The condenser should
be of such a form, and be so placed, as to give the greatest possible spherical aberration. Plano-convex is a very good form, with the plane side next to the light. Some operators put the convex side next to the light, but that is incorrect, because it is required to increase, and not to diminish the spherical aberration of the image of O. It is evident that if the lens D E had no spherical aberra tion, the pencils A m, / C m, 1B m, would become straight lines, and the condenser would be useless.
When sunshine is to be used instead of artificial light, the form of the condenser must be modified, so as to suit the parallel rays inci dent upon it. The construction is shewn in Fig. 13.