SCIENCE-GOSSIP. 



Orthochromatic Photography. — The term 

 " orthochromatic "' photography, or " isochro- 

 matic " photography, is applied to the process 

 of producing ••colour-correct" negatives. Some 

 colours, as we know, are more actinic (photo- 

 ;raphicallv) than others ; thus the dry plate is 

 more sensitive to rays of light reflected by blue 

 and violet objects than it is to red and yellow rays. 

 Yet such vellow rays may appear to the eye 

 brighter than do the blue, if we photograph the 



joectrum upon an ordinary gelatino-bromide 

 plate, a print 

 from the result- 

 ant negative 

 would show that 

 portion repre- 

 sented by the 

 rei section of the 

 spectrum as quite 

 black, the orange 



-'.low nearly 



-.i!e the blue 



3 appear 

 perfectly white 

 and the deeper 

 shades of violet 

 and indigo much 

 lighter than they 

 really were. If 

 v. e photographed 

 together a bunch 



.et-me-nots 

 and primroses the 

 latter would ap- 

 pear much darker 

 than the former, 

 although to the 

 eye the reverse is 



lly the case. 

 due to the 

 fact that yellow 

 are the illumin- 

 ating rays which affect our eyes most, whereas the 

 blue are the chemical or photographic rays, and 

 these almost exclusively act upon the photographic 

 It is. then, to lessen the actinicity of the 

 blue and violet rays and to increase the power of 



eUo» and red rays that orthochromatised 

 plates are prepared, and a colour screen employed 



■no bromide plates may be orthochromatised 



during the mixing of the emulsion, or after 



the plate. In both instances it 



is effected by staining the emulsion with a coal-tar 



lour -generally of the eosin 



l-.o in luelf, erytbrofin, rose Bengal, 



: other shades, can be and often are 



radically impossible to produce 



lates, that i-,, a plate 



■ 



of the 

 spectrum Different dyes 



tpecial claws of .-., if the object to DC 



photograph' .hould 



Relative visual 

 intensity. 



Ditto with eosin. 



: op Effect of Dvf.s. 



be employed ; while rose Bengal and erythrosin 

 sensitive for yellow, and eosin for green and 

 greenish yellow. A very weak solution of the dye 

 is necessary — about one part in 10,000 being 

 sufficient. 



The addition of the dye to the film tends 

 perhaps to decrease the rapidity of the plate, but 

 that can be recovered by the addition of about 

 one per cent, of ammonia to the solution or 

 emulsion. 



The subjoined diagram will show the effect of 

 different dyes when photographing the spectrum, 

 the height of the curves showing the relative 

 degrees of sensitiveness at those parts. 



The dotted lines represent Fraunhofer's lines, 

 viz., a, red ; b, orange ; c, yellcw ; d, green ; e, 

 indigo blue ; /, violet ; and g, ultra violet. 



It will be observed that the application of dyes 

 to the emulsion does not affect its sensitiveness to 

 the blue rays to any appreciable extent in either 

 direction. If it be desired to obtain still greater 



action at the red 

 end of the spect- 

 rum in relation to 

 the sensitiveness 

 of the blue and 

 violet rays, it is 

 necessary to ob- 

 struct the latter, 

 while the red and 

 yellow rays are 

 permitted to 

 reach the sensi- 

 tive plate without 

 hindrance. This 

 is done by impos- 

 ing a transparent 

 yellow screen be- 

 tween object and 

 plate. Such a 

 screen is gener- 

 ally fixed directly 

 in front of the 

 lens or in the lens 

 mount, and is 

 made of glass, 

 with perfectly 

 plain parallel sur- 

 faces, or of a thin 

 film of collodion 

 tinted to the de- 

 sired shade. 

 The use of such a yellow screen greatly pro- 

 longs the time of exposure necessary, as it 

 cuts off a large proportion of the most actinic 

 rays, and its use will be found necessary only in 

 exceptional cases, such as highly-coloured pictures, 

 flowers, etc. 



It is by no means a simple matter to stain coated 

 plates satisfactorily, but many makers now supply 

 orthochromatic plates, the dye being added during 

 the preparation of the emulsion, which are cleaner 

 and '•{ more e'|tial sensitiveness than the photo- 

 grapher could secure by colour-sensitising his own 

 plates. Being so sensitive to yellow and red, 

 othochromati' plates must be manipulated with 

 the utmost care in the dark room. The light 

 employed must be well diffused and of a very 

 deep-red colour; the orange light, which is 

 found perfectly »afe f'>r ordinary plates, will 

 plati hould be exposrd to 

 tbjj light as little a\ |>o: iliN until, at all events, 

 1 i' ed. — C. C. Vevci '., 163, 



Ditto with ery- 

 throsin and rose 

 Bengal. 



Ditto with 

 cyanine. 



