Dr. Maddox^ on Photomicrography . 37 
the illumination to shut out diffused light as much as possible 
from the under surface of the object. It seems preferable to 
have a full-size collecting or posterior lens in the achromatic 
combination^ by which means a large volume of light is made 
to replace^, as it were, the defect of the decrease in angular 
aperture occasioned by the withdrawal of the focus of the 
condenser, either to avoid the sun-spot or injuring the object 
and objective if of very short focus. The illumination is by 
direct sunlight, generally employing Abraham^s achromatic 
prism in place of the mirror if the object be not large. In 
the determination of structure I anticipate better results from 
the use of concentrated parallel rays by achromatic lenses or 
a speculum reflector. On trying the prism and a plano-con- 
vex lens for a condenser, the foci meeting, the resulting 
image was deficient in vigour, and a certain amount of fogging 
very evident. Using sunlight, it is difficult, under long ex- 
posure, to hit the balance between the necessary light and 
the diminishing stops. Although much may be gained by 
the use of oblique illumination in the ordinary employment 
of the microscope, it opens a question in photomicrography 
as to the correctness of the representations, for the inter- 
ference in some objects arising from the obliquity of the 
light may be so great as to furnish a corresponding error. 
This is seen more especially when the oblique pencils, play- 
ing on the edges or angles of the refracting body and being 
possibly repeated, even in the structure by reflection, render 
a single line or marking double or compound. 
I do not here take into account the so-called " blurring'^ 
of photographers, said to arise from the reflection of the 
actinic rays after passing through the collodion film, by 
the back surface of the glass plate, or the indistinctness of 
bordering due to objects lying a little out of focus, and, as 
in the case of fine hairs, also depending on interference. The 
error alluded to is noticed more particularly when the ob- 
ject has a certain thickness, as in some of the Coscinodisci ; 
then it may furnish to the hexagonal areas an appearance as 
if the sides of the depressions or elevations were made up of 
a series of short rods or superimposed planes. This is 
especially the case when the object is focussed into. In some 
of the discs, as in Actinophsenia, a slight obliquity in the 
illumination tends to furnish a bolder contour to the object, 
of value when the pairs are united in the stereoscope. Mr. 
Wenham, some time since, pointed to this instrument as 
likely, through photomicrography, by high- power objectives, 
to render considerable assistance in determining such points 
as are with difficulty, if not uncertainty, interpreted by the 
