3 
Journal of the 
A balancing of the light by which we view an object in this way is 
as needful as our own natural apparatus in the eye, namely, the 
iris , or veil , which divides the eye into an anterior and posterior 
chamber. And if anyone will be at the trouble of watching liis 
neighbour’s eye while he is examining ordinary objects around 
him, or even if he observes his own pupil, or contractile opening 
in this veil, by looking into a mirror, he will perceive that this 
opening is in continual action, dilating or contracting in order to 
adjust the amount of light needed by the eye as it wanders over 
near or far off objects, or variously-coloured objects. The fact is, 
it is a continual focussing of the light for special definite work 
which this organ (the eye) is constantly engaged in ; hence the 
importance of being able to modify the light admitted to an 
object viewed under the microscope. Hence, in working with high 
powers, that is above the quarter-inch objective, on very delicate 
objects, the light must be shut out or let in gradually. The 
action of oblique light under the stage or object partakes of the 
nature of focussing the light, for in such cases, under oblique 
illumination, the eye in great measure sees the object by the light 
reflected from it, and not directly through it. Of course with 
objects requiring low power, it is easy to moderate the light by 
distance and other ordinary contrivances. 
An easily applied method of oblique illumination consists in 
adopting the plan of Dr. Woodwai’d, U.S.A. : Of a small right 
angle prism fastened to the slide by a little glycerine — a little skill 
and patience being requisite to become acquainted with this plan. 
Another is that of adjusting a silver speculum immediately below 
the object, so as to throw the light into the slide from some angle. 
By the use of the silver speculum the light is not confused by 
producing a double reflection, as in the case of light thrown from 
a glass silvered mirror. 
I' must not pass over the subject of diatoms, lest I should 
acquire the character of a diatom hater by that part of the com¬ 
munity of microseopists who are sometimes styled Diatomaniacs, 
because they choose to spend much of their time on such objects. 
The study of these objects in the hands of those who have had 
money and leisure, has been the means of advancing the production 
of excellent objectives, and if any such workers like still to pursuo 
