56 
PRACTICAL PHOTO-MICROGRAPHY. 
on ordinary plates give a blurred image in photo¬ 
graphic work, will give a sharp image when an ortho- 
chromatic plate is used. This improvement is increased 
by the use of such a light filter or screen as will cut off 
more or less of the violet and blue regions of the 
spectrum ; the yellow glasses supplied by the makers 
of “ Isochromatic ” and other colour correct plates are 
found to answer well. Further, certain objectives in¬ 
tended to be used with a projection or other ocular, 
which do not give a sharp image on an ordinary plate 
when the ocular is not used, will give a sharp image on 
an orthochromatic plate under the same conditions. 
Mr. E. M. Nelson finds, moreover, that even the apo- 
chromatics work distinctly better on colour correct 
plates than on ordinary. And Mr. A. A. Carnell has 
demonstrated to the writer that he gets better results 
with balsam-mounted objects, or objects in any of the 
yellow coloured media, than with objects mounted dry. 
Briefly, objectives, so-called achromatic, of old and 
cheap types, will frequently, if not always, give sharp 
photographic negatives on orthochromatic plates, 
though they may not do so on ordinary plates; a 
screen cutting off violet and blue rays “ helps out ” the 
correcting influence of the plate ; and we are to take as 
tantamount to a yellow screen any substance in the 
light-way, or any quality in the illumination, that 
makes the light acting on the plate more or less yellow, 
or green yellow. But these facts do not in any degree 
help us in regard to wide aperture ; the spherical aber¬ 
ration of an objective is not counteracted in any way 
