158 THe Microscope. 
be then reflected downwards to the back lens, it will strike this 
lens in the middle and will finally reach the object as more or 
less central light. Butifit be admitted to the edge of the reflect- 
or it will then pass through the edge of the back lens, and when 
it reaches the object will be an oblique ray to the extent of half 
the angular aperture of the objective. When admitted through 
the entire opening, the light will reach the object as a cone whose 
angular breadth will correspond with that of the whole angular 
aperture of the objective. 
The conditions regulating available aperture, hold good with 
verticle illumination as well as with other forms. If an object- 
ive whose aperture exceeds the numerical value of 1.00, be used 
upon a dry-mounted object, the aperture will be reduced, in ef- 
fectiveness, to that value—thatis to say, it will only equal a dry- 
working objective whose angular aperture is 180°. On the other 
hand, if the object be mounted in balsam, although the full ap- 
erture will become available, the visibility of such objects as di- 
atoms will become very much reduced. Thus it is that resolu- 
tions are frequently accomplished with a reduction in strength 
of image. But this condition need not be produced at all, for 
we now have mounting media whose indices of refraction are 
sufficiently high to render a visibility equal to, and in some cases 
greatly exceeding that of a dry mount. Among the media of 
highest indices are the solution of phosphorus in bisulphide of 
carbon as recommended by Mr. J. W. Stevenson, having an in- 
dex of 2.10; the colorless medium of Prof. H. L. Smith, which 
closely approaches 2.20, and my own medium (a sulphide of 
arsenic) having an index of 2.33, as determined by working it 
into a prism. Any of these media will give results with verti- 
cal illumination which are far better than was thought possible 
a few years ago. 
But there is another medium to which I wish to call atten- 
tion, I have used it considerably during the past few months and 
find it far ahead of anything I have yet used. It is pure silver. 
By Brewster it is stated that the refractive index of silver is 3.27. 
One side of the diatom is burned to the cover-glass and the other 
side is then coated with the silver. The visibility of a diatom so 
prepared is four times as great as when dry mounted, or more cor- 
rectly, in the proportion of 1.84 to .43! The results obtained by 
