"The original construction was of very great power or magnification, but Mr. Dallmeyer has subsequently introduced instruments of more moderate power. The high power systems include an angle to is degrees; and the latter more moderate power from i8 to 20 degrees.
" Before giving shortly details of the method of calculating the focus and intensity ratios of the telephotographic system, we must call photographers' attention to two very important factors in the employment of this instrument.
" In the first place as we have already stated the optical center of the combination being out side and beyond the front combina tion. it will be evident that the slightest tremor in the camera itself will appear very much ex aggerated in the resulting image as against using an o rdi n ar y camera, in which the optical center is contained within the lens itself, for in the telephoto graphic lens we have practically to consider any 'slight movement in the camera itself as a movement of the short end of a lever, the in distinctness of the image being reckoned by the vibration or move ment of the long arm of the lever (in our case the optical center situ ated outside the camera).
"The other point to be taken into consideration is one that can only be arrived at by practical ex perience, and that is the very short exposures necessary for distant _ objects. In ordinary photography we usually find without very care ful development that the distance is "burnt out" if a full foreground development is required. No definite rule fof reduction and exposure for distant objects can be set down, but if the exposure is calculated according to inten sity ratios, we shall be safe in stating that a reduction of one half to a third of that calculated in the ordinary manner may be given.
" The simplest means for determining the corresponding foci and intensity ratios of a tele photographic system is by means of the ratio between the foci of the positive and negative ele ments. This ratio can always be determined; for example, if a six-inch lens is combined with a three-inch negative. the ratio is 2 to I ; if the six-inch positive is combined with a two-inch nega tive, the ratio is 3 to 1, and so on. If the distance between the diaphragm of the lens and the
screen be measured and multiplied by the ratio as above stated, the result will be the equival ent focus of the system, and the inten sity is simply this equivalent focus, divided by the clear aperture of the diaphragm employed. This rule is quite sufficiently accurate for all ordinary pur poses, but we will add a further and exact mathematical rule, dependent upon the linear magnification of the image given by the positive lens alone.
" Magnificatzon.—Divide distance from negative or posterior lens (after the image has been focused sharply) to focusing screen by the focus of the negative lens, and add one to the result. "Corresponding focus.—Focus of positive lens multiplied by the magnification. " Intensity ratio.—Intensity of positive divided by the magnification." TEMPERATURE.—The temperature of the atmosphere has undoubtedly an effect upon photographic operations. Intense cold opposes the formation of the latent image while intense heat will accelerate it. Every photog rapher is aware of the importance of keeping solutions at a fixed tempera ture. An ingenious arrangement for keeping developing solutions at a fixed temperature, devised by C. E. Hearson, is shown in Fig. 463.
The apparatus contains two 12 feet lengths of tubing (NN, 00), the water circulating through one tube (N) and the developer through the other (0, Fig. 463). The water is first passed through and received in the develop ing dish, which is thus warmed, the dish being then empty to receive the developer, both being at the same temperature. The thermostat (D) con sists of two pieces of sheet copper soldered together at the edges; be tween the copper is placed a piece of blotting paper saturated with a liquid, which in this case boils at about 70 deg., so that so long as the liquid with which the paper is saturated is not hot enough to boil, the two sides of the copper capsule would lie quite close together, but when the critical tem perature was reached, the sides would be distended, and a central rod (K) pushed up into the gas valve (H) and the height of the flame (B) thus adjusted.