The following table gives the sizes and particulars of carbons for various currents that I have found best in actual practice:
| Continuous Current | ||
| Ampères. | + Carbon Cored. | - Carbon Solid. |
| 7-10 | 12 mm. | 7 mm. |
| 10-15 | 13 ,, | 8 ,, |
| 15-20 | 16 ,, | 10 ,, |
| Alternating Current | ||
| Ampères. | Upper Carbon Cored. | Lower Carbon Solid. |
| 7-10 | 12 mm. | 10 mm. |
| 10-15 | 13 ,, | 11 ,, |
| 15-20 | 16 ,, | 13 ,, |
CHAPTER VII
THE OPTICAL SYSTEM OF A LANTERN
As previously noted, the essential parts of an Optical Lantern are, in order from rear to front: (1) The illuminant; (2) the condenser; (3) the slide and slide stage; (4) the objective, to which must be added, (5) the body or framework which holds the whole together. Fig. 33 is a diagrammatic representation of the entire optical system and Fig. 33A shows all the various parts in situ: A being the illuminant, shown in Fig. 33
as an arc lamp, B the condenser, C the slide stage, and D the objective. The foundation, so to speak, of the whole instrument is of course the slide, which, as made in this country, consists of a square of glass 3¼ inches diameter, the slide itself being somewhat less than this on account of the binding, &c.; in making calculations it is usually taken as a 3-inch circle. Slides are usually made by binding together with strips of paper or cloth two such squares, on one of which is the photographic film or painting forming the picture, the other being simply a plain cover glass placed over the slide surface to protect it, and between the two being placed a paper mask with an aperture of whatever size or shape is required, that of the aforesaid 3-inch circle being usually taken as the standard or normal dimension for this aperture.
The slide being illuminated by one of the various methods discussed in the previous chapters, is focussed on the screen by the objective, which must be selected according to the size of picture required and the distance between lantern and screen.
These points will be gone into later, and also details as to various types of objectives and their respective advantages; but it may be said here that a lantern objective consists usually of a combination of lenses of 2 inches or 2½ inches diameter mounted in a rackwork focussing system at a distance from the slide of 6 inches to 18 inches, according to the length of its 'focus.' As our slide is from 3 to 3¼ inches diameter, it is evident that all the light radiating from this cannot possibly get through the objective unless it is converged upon it, and to do this is the function of the condenser. The following two diagrams, Figs. 34 and 35, will make the matter clear.