THE LIGHT. 39 
technical difficulties which have not been solved 
in a satisfactory manner. 
Photo-micrographs of the surfaces of opaque 
objects, of insects, foraminifera, etc., would be ex- 
ceedingly interesting, and useful for illustrating 
scientific treatises, popular lectures, etc. 
It is well known, in ordinary photography, that 
objects in different planes can only be brought 
into focus at the same time in a greatly reduced 
picture. In a life-size photograph of a man, for 
example, when the nose is in perfect focus, the 
ears will be out of focus, indistinct and distorted. 
This difficulty is overcome by making a reduced 
picture in the first instance, and then enlarging 
this to any extent desired. Minute objects present 
irregularities of form and of surface as great, rela- 
tively, as larger ones, and the difficulty referred 
to is magnified in proportion as we magnify the 
object. 
The quality required of a lens which is best 
adapted to overcome this difficulty is called pene- 
trating power. This quality increases with the focal 
distance of the objective. For this kind of work, 
therefore, it will be necessary to use low-power 
objectives, and those of comparatively small an- 
gular aperture. It will, no doubt, be found also 
that the best results are obtained by making a 
first-class negative with very moderate amplifica- 
tion, and enlarging from this to the dimensions 
desired. 
In the case of flat surfaces, such as a butter- 
fly’s wing, the leaves of plants, etc., the difficulty 
