Wen HAM, on Microscopic Photography. 5 
termed " over corrected ;" for the point of convergence of the 
chemical rays near the most refrangible or blue end of the 
spectrum, lies beyond the visual focus. I have an objective 
corrected specially for these rays, and though perfect for pho- 
tographic purposes, yet on account of its being non-achromatic, 
it is unsuited for microscopic investigation. 
The simplest and cheapest way of producing the required 
degree of " under correction," is to screw a biconvex lens into 
the place of the back stop of the object-glass, acting as part 
of its optical combination For Smith and Beck's 1^ inch, I 
have used a lens of 8 inches focus, and for the 2-3rd, one of 
5 inches, which also serves for the 4-lOth inch ; these bring 
back the actinic to the exact position of the visual focus. 
The combination used as an objective shows some colour, but 
the additional lens does not affect the spherical correction to 
any material extent, and the increase of distinctness in the 
photographs obtained by means of the application of this 
additional lens is most striking. 
It must not be supposed that the focal lengths that I have 
here given of the correcting lenses will in all instances serve 
for the objectives above named, for the correction will probably 
require a lens of a different focus for every different object- 
glass ; it is, perhaps, best to be provided with several of them, 
as their cost is but trifling. Those that I have made use of 
have been selected from ordinary spectacle lenses, most care- 
fully centered and turned down to the required size. 
Some remarks have been published on the possibility of 
obtaining stereoscopic pictures of microscopic objects by 
means of the " binocular microscope," but the ordinary in- 
strument will answer every purpose without any optical 
addition whatever, for it has been shown* that if the object 
itself be viewed alternately with the right and left half of the 
object-glass without any altering of its position, the difference 
in form of the resulting images assimilates to the effect of a 
different angle of vision ; and if two photographs of the object 
obtained by the separate halves of the object-glass be placed 
in the stereoscope, they will give an appearance of solidity 
to the object. The only addition that is required to produce 
this effect is to fix a sliding stop close behind the objective, 
having straight edges that will cut off either the right or left 
hand sides ; a photographic impression being taken at the two 
extremes. If the object-glass be one of considerable aperture, 
about one-third only of the diameter may be cut off, which 
will be sufficient to give the difference of form required for the 
stereoscopic image. 
* ' Quarterly Journal of Microscopical Science,' for July, 1853. 
