PHOTOGRAPHY 



rendering of the green foliage and sliglitly increasing 

 the trutii of representation in pink, lavender, and the 

 lighter red shades. It is very much better, then, for the 

 photographer who has to do with horticultural \V(irk to 

 confine himself exclusively to these isochi-oinatii- jilates 

 for all his work. If he has a suliji-ct iiiclu.liii!,' hliio 

 flowers, the especial activity of the blue rays, whi<'h 

 otherwise would render the photographic impression ;is 

 intense as if the oliject was white, can be rc.straine<l by 

 a suitable ray-filter, which is applied in front of the lens. 

 This ray-filter is either a glass cell filled with a one per 

 cent solution of potassium bichromate, or a jiiece of 

 plane optical glass covered with a suitably stained col- 

 lodion film sealed with another optical glass and pro- 

 vided with a convenient mounting for slipping on the 

 lens. With this ray-filter and the isochromatic plate be- 

 fore alluded to, the yellow is slightly over-valued, but 

 the blue is given its proper relation. The beauty of 

 outdoor photographs is vastly increased by the use of 

 the plate and ray-filter mentioned, because a proper 

 color value is given to the sky, and the cloud forms are 

 preserved in all their attractiveness. If the subject be 



PHOTOGRAPHY 



i;ji5 



cf the isochromatic plate as compared with the ordi- 

 nary plate in photographing yellow tulips.) Halation is 

 caused by the reflection of brilliantly lighted objects 

 from the back of the glass plate carrying the sensitive 

 emulsion. Light possesses riiorinnus velocity, and 

 there is a constant and aliih.st iiir.iiic. ivably rapid play 

 back and forth between the tw.i suiiaces of the glass 

 lilate, which is covered only on its face by the sensitive 

 lilrotographic emulsion. This results in a thickening of 

 all the finer lines which should be rendered in the posi- 

 tive as white. It is best counteracted by "backing" the 

 plate with a composition which will absorb all the rays 

 of light that p«ss through the einulsion on the face. 



1768. AdjustaDle camera-stand, horizontal position. 



a heavily loaded peach tree, for instance, the accentua- 

 tion of the yellow, brought about by the use of the ray- 

 filter, will give a needed slight exaggeration of color 

 value to the fruit, which, under treatment by an ordinary 

 plate, will be almost indistinguishable from the mass of 

 foliage. With the ray -filter the exposure required is 

 practically trebled. In this Cyclopedia, advantage has 

 been taken of isochromatic plates in photographing 

 some of the subjects. The carnations, Plate IV, Vol. I, 

 show a variety of shades properly rendered by the 

 means indicated. The dahlia, Plate IX, shows the 

 rendering of dark red shades in connection with green 

 foliage by the isochromatic plate. The persimmon 

 plate (XXVIII) in this volume also shows orange, yel- 

 low and red fruits as properly rendered under this 

 isochromatic handling, and the oranges (Plate XXIII), 

 including fruits from the light lemon-yellow Pomelo to 

 the deep orange-red Tangierine, prove also the useful- 

 ness of the plate suggested. 



A difficulty known in photographic practice as hnln- 

 tion must also be counteracted if the fine detail of sub- 

 jects involving much light is to be preserved. Plate 

 XXIX shows a milkweed pod, first as photographed witli 

 a plain isochromatic plate and second as photographed 

 with the same plate, treated so as to eliminate halation. 

 (The two lower figures on the same plate show the vabii- 



1769. AdjuaLdOle cainera-stand, vertical pasi,i..,i. 



Any dark substance which can be brought into absolute 

 optical contact with the pusteriur siiilan- of the glass 

 plate will answer, but tin- mosi illicii nt hai'kiiig is 

 provided by an alcoholic sulniicm .if sciap, t" wiiich has 

 been added a mixture of i rythnisin and uurin, two red 

 aniline pigments having high absorptive powers. This 

 coating is easily applied, and easily removed before 

 development; and the writer in his practice of Horti- 

 cultural Photography uses isochromatic plates backed 

 with this dyed soap-solution for all important work. 



Even with the aid of the isochromatic plate and the 

 ray-fllter the photography of shades of red is diflicult. 

 because of the lack of actinic or chemical quality in the 

 red rays of the spectrum. In practice it is found neces- 

 sary to give a very much prolonged exposure to objects 

 containing red, and then to restrain the overexposure 

 upon development by means of a suitably compounded 

 developing solution. Details regarding this are out of 

 place here, but may be obtained from the works cited at 

 the end of this sketch. 



LaHtern-xlides.—A few words may be added concern- 

 ing the production of lantern-slides, now of the greatest 

 importance in illustrative work. A lantern-slide is a 

 positive on glass, and therefore is made from a nega- 

 tive. It is made preferably on a special plate, much 

 slower than the regular photographic dry-plate, because 



