. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. * A description of the woodcuts in Chapters L, LI and LII will be found inChapter LIII. 466 PHOTOMICROGRAPHY. The photographic processes used are those ordinarilyemployed in practice; it is only necessary to mention thatthe development by pyrogallic acid alone is often prefer-able, owing to the uncertainty there exists as to the exactintensity of the light. The principal difficulties are in The focusing requires the greatest precision, the focusof the objective being so very sho
. The silver sunbeam : a practical and theoretical text-book on sun drawing and photographic printing. * A description of the woodcuts in Chapters L, LI and LII will be found inChapter LIII. 466 PHOTOMICROGRAPHY. The photographic processes used are those ordinarilyemployed in practice; it is only necessary to mention thatthe development by pyrogallic acid alone is often prefer-able, owing to the uncertainty there exists as to the exactintensity of the light. The principal difficulties are in The focusing requires the greatest precision, the focusof the objective being so very short; it is commenced withthe coarse, and finished with the fine adjustment, and amagnifying glass applied upon the ground glass. Whenthe length of the apparatus is very great, it is necessary tohave recourse to an assistant, who acts according to theinstruction of the observer. On the exposure depends, in a great measure, the valueof the proof. The direct solar light can alone unite the necessary photographic quali-ties; the diffused light is toofeeble in most cases to be ofany utility. The artificial ex-posure by electric light, mag-nesium, gas, or Drummondslight have only an experimentalinterest, and are unsuited tothe usual exigencies of practice. It is necessary to have a pureand homogeneous light, freefrom any inactive parts, uponthe sensitive surfaces. The so-lar rays generally come through vapors suspended in theatmosphere; they have then, when reflected, a yellowishtint detrimental to the p
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