Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/872

 836 PHOTOGRAPHY Fro. 1. Spectrum Effects on Salts of Silver. [P. = print; D. = developed ; I.e. = long exposure ; s.e. = short H AC F D CBA t 4 . 5 2 P. P. ^ &quot;]&* Agl on paper washed from P. excess of AgXO 3 and treated with KXO 3 s ^ &quot;k, Agl on paper washed from P. AgXO 3, soaked in NaCl, washed from excess, and e _ exposed with KXO Paper floated on AgXO 3. . P. ,^_ 7 __^&quot; - Agl on paper washed from P. excess of AgXO 3, ruddy tint 8 _- -*&quot;&quot; -_ Agl on paper washed from P. excess of AgXO 3, treated with KI and KXO 2 ; or Agl 9 ^^ - in collodion AgI+AgXO 3 in albumen .. P. _^- -_ Agl prepared in bath, treated D. with KI, washed, redipped (I.e.) in silver bath, developed II ^ ^ with pyrogallic acid ,, ,, (s.e.) , 2 ^ &quot; Agl purified and exposed in D. presence of sensitizer, de- (I.e.) veloped by acid or alkaline 13 -- ^~ . developer ,, .. (3 e ) Agl unpurified, treated, and D. developed as above. . (1 e ) __^&quot; &quot;^_ 15 _ ^ &quot;V^ Agl with trace of AgCl or D. AgBr, developed by acid (I.e.) or alkaline method ,, ,, (s.e.) 17 ^ ^^ AgI+AgXO 3 in albuminized D. collodion, or on paper washed, acid development A gl + A gXO 3 in albuminized D. collodion, or on paper washed, ferrous citrate developer AgI+AgXO 3, prolonged ex- D. posure AgBr+AgX0 3 on paper. . P. .. P. . &amp;gt;. --P. Green AgBr in collodion, with P. or without AgXO 3 Orange AgBr in collodion P. gelatin, with or without AgX0 3 Grey AgBr in gelatin P. .8 _ --- &quot; ~^_ =- - i 19 .^ - &quot; ^. 20 _-^^ 2, _- &quot;^ _ | fe 22 .--- &quot;&quot;&quot;&quot; - -_ 23 1 ~ 24. ___--- v^ 25 ____- -&quot;&quot; x 26 _. x&quot; AgBr on paper washed from D. AgXO 3, acid or ferrous (I.e.) citro-oxalate developer ,, ... (ae) 27 ___ -&quot;&quot;^&quot;

28 ^&quot; &amp;gt; Grey AgBr in gelatin, de- D. veloped alkaline or ferrous (I.e.) t) oxalate ,, ( a e ) v 29 **&quot; s 30 _^- &quot; -V Orange AgBr in collodion or D. gelatin, alkaline ferrous (I.e.) n oxalate or acid developer v, (8.8.) T 3, ^-^1 &amp;gt; 32 _/ - ^ Effect of Dyes on Sensitive Films. In 1874 Dr Vogel of Berlin called attention to this sub ject. He found that when films were stained with certain aniline and other dyes and exposed to the spectrum an increased action on develop ment was shown in those parts of the spectrum which the dye absorbed. The dyes which pro duced this action he called &quot;optical sensitizers,&quot; whilst preservatives which absorbed the halogen liberated by light he called &quot;chemical sensi tizers.&quot; A dye might, according to him, be an optical and a chemical sensitizer. He further claimed that, if a film were prepared in which the haloid soluble salt was in excess and then dyed, no action took place unless some &quot; chemi cal sensitizer &quot; were present. The term &quot; optical sensitizer &quot; seems a misnomer, since it is meant to imply that it renders the salts of silver sensi tive to those regions of the spectrum to which they were previously insensitive, merely by the addition of the dye. The idea of the action of dyes was at first combated by many, but it was soon recognized that such an action did really exist. Abney showed in 1875 that certain dyes combined with silver and formed true coloured organic salts of silver which were sensi tive to light ; and Dr Amory went so far as to take a spectrum on a combination of silver with cosine, which was one of the dyes experimented upon by Major Waterhouse, who had closely followed Dr Vogel, and proved that the spectrum acted simply on those parts which were absorbed by the compound. Abney further demonstrated that, in many cases at all events, the dyes were themselves reduced by light, thus acting as nuclei on which the silver could be deposited. He further showed that even when the haloid soluble salt Avas in excess the same character of was in excess, though the exposure had to be jrolonged. This action he concluded was due to the action of the dye. The subject has been discussed again recently owing to the production of so-called iso-chromatic films, i.e., films which are_ supposed to be sensitive to all colours, and which are prepared on gelatin or collodion plates 3y dyeing them with cosine or some similar dye ; and the instructions given indicate that, if a
 * pectrum was produced as when the silver nitrate
 * oloured picture or landscape be photographed

hrough yellow glass, the &quot;yellows&quot; will be lenser in the negative than will the &quot;blues.&quot; Experiment shows if a film after preparation be lipped in a solution of &quot;eoside of silver,&quot; made ry precipitating cosine with silver nitrate, wash- ng the precipitate, and then dissolving in water aintly alkaline, a negative taken in the usual vay will give the &quot;yellows&quot; equally as dense as he &quot;blues.&quot; The action of the yellow glass is o cut off the blue rays to which the normal alt is most sensitive, and to leave the yellow ays unaltered ; these then expend their energy ipon the organic salt of silver. The advantage of rendering the yellows of a picture most in tense in a negative is that the resulting print will be more nearly true to nature, since these are the most luminous rays. Further experi ment ought surely to show how this can be done without the introduction of the tinted glass. Action of the Spectrum on Chromic Salts. The salts most usually employed in photography Dyes an sensitivi films&amp;lt;