The Present Limits of Vision. By Dr. Royston-Pigott. Ill 
The miniature landscape disappeared. But in shadow very minute 
details of the miniature house came sharply into view. 
Experiment 2. — A mercurial thermometer bulb about three- 
quarters of an inch in diameter was placed at fifty yards distance. 
In shadow a very fine telescope enabled me to descry the miniature 
of trees and the chimneys of the house against the sky. In sun- 
shine the diffraction hid all these details. 
If a long line of light be similarly observed, such as a brightly 
reflecting steel rod, the diffractions follow accurately the outline of 
the rod, and elegantly turn round the ends with their exquisite 
diffraction rings beautifully defined according to the quality and 
power of the telescope. 
The ^ inch lens appeared to give the neatest disk ; the larger 
lenses gave too large an image of the sun, and spoiled the diffrac- 
tion phenomena. Still better than reflected light is transmitted. 
For when a prism is so placed as to give total internal reflexion, 
the miniature of the sun assumes a surpassing splendour, appearing 
like a veritable electric light of overpowering brilliance. A lens 
of J inch focal length then gives a solar disk of 4 Jo of an inch 
/ 30 
nearly in diameter \A sin. 30' nearly = i x ^^gg , since 1' nearly 
= g^^^- when even a lens of 2V is used, the disk is the 
same size, only less bright. In that case the size of the disk is 
or the 2 oVo of an inch nearly. Now this placed at fifty yards 
would be 0"-0573, or XT of a second, impossible to be seen by 
any telescope in existence. This again shows the tremendous 
expansion of a point of light, being an exact miniature of the sun 
■2 oVo in diameter. For it appeared quite a considerable disk, fully 
one-sixth of an inch in diameter, or at least three hundred times 
its proper size. 
The optical instrumentation by which these disks are presented 
to the eye by the eye-piece may be of many kinds—a long tube, as 
in the telescope, or a short one, as in the microscope : the effect is 
precisely the same for all. The eye indeed may be covered with a 
minute aperture, and look directly (but nearly) at a brilliant point 
of light : in each case the disk is expanded into considerable dimen- 
sions with its accompanying rings. In using these disks with the 
microscope, the instrument is set horizontally, so that the |th 
object-glass used as a condenser forms a minute brilliant miniature 
disk at 100 inches distance. My immersion ^th is really xVth, and 
the image is diminished one thousand times ; the theoretical image of 
