46 THE MICROSCOPICAL NEWS. 
materials from which lenses are made, yet it introduces many com- 
plications, which, affecting as they do the manipulation of the 
microscope, should be clearly understood by every working micros- 
copist. 
It has often been said in this science, as with many other 
things, that theory and practice will not always row together ; but 
I wish to remark that, where practice does not conform to science, 
it will be found on searching deeper into our subject, that we have 
forgotten some necessary quantity, or have been ignorant of some 
hidden theory, essential to completely explain apparently contra- 
dictory results. 
In speaking to you of refraction, we must put it in its simplest 
form, which is when the medium traversed by a ray of light is 
furnished with parallel sides, such as we shall meet with daily as the 
3 by 1 glass slip, or an ordinary thin glass cover. 
Mr. Blackburn, in his paper on “‘ The Theory of Aperture,” put 
before you very clearly what is meant by refraction. Every 
medium into which a ray of light can enter has an effect upon it, 
causing it to bend from the straight line, various substances deflect 
the ray more or less; but it will be found that in the same media 
the angles of incidence and the angles of refraction when com- 
pared by their sines, always exhibit a constant ratio. The more 
perpendicularly the ray strikes the medium, the less is its actual 
deflection ; the more obliquely, the greater the amount scattered 
by reflection. 
Now different media exert a more or less greater influence upon 
the luminous ray, and in order to show you this I have prepared a 
diagram, illustrating how a ray of the same obliquity in air, im- 
pinging upon layers of glass, water and glycerine is variously 
deflected. I wish you to bear in mind that the air ray in each 
case is the same, and the various angles in the denser media,—all 
and each correspond to the obliquity of the impinging air-ray. This 
is a very important fact, and I shall have to refer to it again by- 
and-by ; but you will remember that Mr. Blackburn told us more 
than this, he showed in a very clear manner that the sines of the 
angles of refraction, and those of the angles of incidence, always 
possessed a certain ratio to each other, so that we can always pre- 
dict what course a ray of any obliquity will take. 
I have illustrated this subject by taking a ray of such obliquity 
as will just pass through a glass plate with parallel sides. If we in- 
crease the obliquity of the ray it will, to a certain extent, still enter 
the glass, but when the inside angle has reached 42° the ray will 
not pass out of the upper surface, but will be totally reflected from 
that surface downward. I wish you to remark that this total 
reflection takes place iz the denser medium—tfrom the rarer medium 
to the denser, such as from air into glass a portion of the rays of 
