520 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and species, thanks to the comparison alone of the intimate structure of 
the fibres and cellular network. By this means we are enabled, with 
the help of simply thin cuttings, to give, so to say, a complete anatomy 
of each species, and to notice easily the essential differences which exist 
between woods of different species, although belonging to the same 
family ; all the more, therefore, can we recognize classification in 
families. Microphotographic pictures, projected by the lantern, served 
to demonstrate clearly the truth of the propositions affirmed. This is a 
new example of the numerous services that photography may render to 
the sciences. 
The Coloured Screen in Photomicrography.* — The following is an 
abstract of a paper by Professor Romyn Hitchcock : — 
An ordinary gelatino-bromide plate is sensitive to the spectrum of sun- 
light from a point between the Fraunhofer lines E and F to about K. The 
maximum photographic action is about G. By considerably prolonging 
the time of exposure the limit of photographic action at the red end of 
the spectrum is greatly extended. In practice the light below the green 
of the spectrum may be regarded as quite inactive when we take photo- 
graphs with ordinary plates. 
By introducing a coloured screen — a plate of yellow glass for example 
— in the path of the light, we may absorb the more active rays, and 
prolong the time of exposure until the yellow rays have time to act 
upon the sensitive plate. In practice, however, it is found that there 
are two difficulties about this method of procedure ; first, in obtaining 
a satisfactory screen, and second, in the long exposure necessary when 
working with the comparatively inactive rays. 
With colour-sensitive plates, such as are now in general use abroad 
and gradually being introduced in this country, the range of photographic 
action towards the red is greatly extended. With such plates the 
yellow screen can be used with great advantage. 
A few years since it was customary to work with monochromatic 
blue light in photomicrography, and the ammonio-sulphate of copper 
blue cell was much in use. When colour-sensitive plates were introduced 
yellow screens took the place of blue, because it was found that many 
specimens had yellow and red and brown parts which were not well 
photographed with blue light. 
The colour and thickness of the screen both require attention. If it 
be too thin the blue light is not sufficiently cut off. In particular cases 
an almost monochromatic yellow light is desirable, as when it is desired 
to obtain sharp outlines of deeply stained objects regardless of 
structural details. But generally a rather broader spectrum range is 
desirable, for the light employed should correspond to the different 
colours or shades of colour of the object. It is owing to neglect of this 
consideration that we often see photomicrographs which are mere 
silhouettes, while the objects show much more structure to the eye. 
This is frequently observed in photographs of such structures as the 
tongue and sting of a bee, and legs of insects. In other preparations, in 
which the colour is a stain, brown or red for example, the fault lies partly 
in the exposure, which, in many cases, is insufficient to give more than 
* Amer. Mon. Micr. Journ., xi. (1890) p. 8. 
