81. PHOTOGRAPHY.--A photographic camera consists of a lens and a device for holding at its focus a specially prepared plate or film. This plate carries a chemical deposit which is very sensitive to the action of light, and which may be made to preserve the imprint of any picture which the lens forms upon it. If such a sensitive plate is placed at the focus of a reflecting telescope, the combination becomes a camera available for astronomical photography, and at the present time the tendency is strong in nearly every branch of astronomical research to substitute the sensitive plate in place of the observer at a telescope. A refracting telescope may also be used for astronomical photography, and is very much used, but some complications occur here on account of the resolution of the light into its constituent colors in passing through the objective. Fig. 44 shows such a telescope, or rather two telescopes, one photographic, the other visual, supported side by side upon the same equatorial mounting. [Illustration: FIG. 44.--Photographic telescope of the Paris Observatory.] One of the great advantages of photography is found in connection with what is called-- 82. PERSONAL EQUATION.--It is a remarkable fact, first investigated by the German astronomer Bessel, three quarters of a century ago, that where extreme accuracy is required the human senses can not be implicitly relied upon. The most skillful observers will not agree exactly in their measurement of an angle or in estimating the exact instant at which a star crossed the meridian; the most skillful artists can not draw identical pictures of the same object, etc. These minor deceptions of the senses are included in the term _personal equation_, which is a famous phrase in astronomy, denoting that the observations of any given person require to be corrected by means of some equation involving his personality. General health, digestion, nerves, fatigue, all influence the personal equation, and it was in reference to such matters that one of the most eminent of living astronomers has given this description of his habits of observing: "In order to avoid every physiological disturbance, I have adopted the rule to abstain for one or two hours before commencing observations from every laborious occupation; never to go to the telescope with stomach loaded with food; to abstain from everything which could affect the nervous system, from narcotics and alcohol, and especially from the abuse of coffee, which I have found to be exceedingly prejudicial to the accuracy of observation."[3] A regimen suggestive of preparation for an athletic contest rather than for the more quiet labors of an astronomer. [3] Schiaparelli, Osservazioni sulle Stelle Doppie.