PHOTOGRAPHY 
rendering of the green foliage and slightly increasing 
the truth 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 work to 
confine himself exclusively to these isochromatic plates 
for all his work. If he has a subject including blue 
flowers, the especial activity of the blue rays, which 
otherwise would render the photographic impression as 
intense as if the object was white, can be restrained 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 piece 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-fllter 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  
1315  
of 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 enormous velocity, and  
there is a constant and almost inconceivably rapid play  
back and forth between the two surfaces of the glass  
plate, which is covered only on its face by the sensitive  
photographic 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 pess through the emulsion on the face. 
1768. adjustable 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 hala- 
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 with 
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 value 
1769 Adjustable camera-stand. vertical position. 
Any dark substance which can be brought into absolute 
optical contact with the posterior surface of the glass 
plate will answer but the most eflitient backing is 
provided by an alcoholic solution of soap to which has 
been added a mixture of erythrosin and aurin, 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 difhcult. 
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. 
lantern-slides.-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 
