20 
sidered to be by reading a formidable work on optics. In alluding 
to the phenomenon of refraction, the lecturer explained, by the aid 
of diagrams, that it was the effect which transparent mediums pro- 
duced on light in its passage through them. Opaque bodies re- 
flected the rays, and transparent bodies transmitted them; but it 
was found that if a ray in passing from one medium into another of 
different density, fell obliquely, it was turned out of its course. 
In passing through a pane of glass, the rays suffered two refrac- 
tions, which, being in contrary directions, produced nearly the same 
effect as if no refraction had taken place; but in plane mirrors 
the rays of light were reflected from objects placed before them, 
exhibiting to us their image. The lecturer then referred to the 
lenses of a microscope. ‘The property of those which had a con- 
vex surface was to collect the rays of light to a focus; and _ those 
which had a concave surface to disperse them. Lenses which had 
one side flat, and the other concave or convex, were called plano- 
convex and plano-concave. The focus of the former was at the 
distance of the diameter of a sphere of which the convex surface 
of the lens formed a portion. Parallel rays were brought to a focus 
by the plano-convex lens at a point. A concaye lens, so far from 
possessing magnifying power, if anything, rendered objects smaller. 
The single microscope consisted simply of a convex lens, in, the 
focus of which the object was placed, and through which it was 
viewed ; but a double microscope was a more complicated instru- 
ment, in which you looked, not directly at the object, but at a 
magnified image of it. In this microscope two lenses were employed : 
the one was placed so near the object that the image which it 
formed was farther from the lens than the object itself was; the 
image, therefore, was larger than the object itself, and it was 
further magnified by being viewed through another lens, which 
acted on the principle of the single microscope, and was called the 
eye-glass. ‘The pocket lens, he said, was most useful and indis- 
pensable to botanists, naturalists, and students of other sciences. 
The proper use of these lenses was to place the eye and the object 
at precisely the same distance from the glass. For instance, if it 
was an inch glass the eye must be an inch from the glass, and the 
object also an inch from the glass, and so on with respect to larger 
or smaller glasses. The lecturer then explained the difficulties 
which opticians encountered in getting rid of spherical and chro- 
matic aberration. Spherical aberration, he said, may be known by 
the want of sharpness when a ftne line, or small spot, or body with a 
well-defined circular line is examined. Instead of the lines appear- 
ing sharp and distinct and definite, they seemed to be blurred and 
foggy, even when focussed with the utmost care; and when there 
are several lines or spots near to one another, they appear to run 
together, producing a general shadow, instead of each one being 
distinctly defined and separated from its neighbours. If the glass 
