. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. PHOTOMICROGRAPHY. 467 defect, the light should be decomposed by passing itthrough an agent which keeps back the inactive part andonly allows the spectral rays to penetrate, which are themost useful for the impression. Count Castracan was thefirst who employed a prism, which acts by dispersion, buthas the disadvantage of absorbing a great quantity of Woodward, and afterward M. Moitessier, made use ofa vessel faced with two parallel glasses, containing a solu-tion of sulphate of coppe
. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. PHOTOMICROGRAPHY. 467 defect, the light should be decomposed by passing itthrough an agent which keeps back the inactive part andonly allows the spectral rays to penetrate, which are themost useful for the impression. Count Castracan was thefirst who employed a prism, which acts by dispersion, buthas the disadvantage of absorbing a great quantity of Woodward, and afterward M. Moitessier, made use ofa vessel faced with two parallel glasses, containing a solu-tion of sulphate of copper and ammonia. This is the mostadvantageous corrective employed for rendering light hom-ogeneous, but it is often with great trouble and difficultyinstalled in its interposition on the course of the luminous Fig. rays. As a substitute, a very pure cobalt-blue glass may beused, which is easily placed on the plate of the it does not possess the same qualities as the so-lution of sulphate of copper, it is more practical. Such isthe opinion of Dr. Maddox, who is an incontestable au-thority on photomicrography. For the minor enlargements the manipulations do notpresent any serious obstacles; the lighting is easy and thequantity of light always suf-ficient. But when a power-ful objective is made use of,the front lens of which isnot a millemetre in diam-eter, and on which the lu-minous rays, after their rec-tification by the cobalt glass,are condensed, there is agreat loss of intensity, especially when combined with alength of camera extended to two metres. It is necessary,then, to condense the solar light by means of several con-verging lenses placed upon its course.
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