TRANSACTIONS OF THE SECTIONS. 17 
any tendency which there might have been to fancy that at double the distance 
double the width of the lines was requisite to admit of resolution. 
As the result of the examination of the sight of nine individuals, it was found 
that seven have equal focus for both eyes; four have longer focus for vertica 
lines than for horizontal; two have shorter focus for vertical, and three have equal 
foci for both. Of seven observers, six have the power of varying the focus for both 
horizontal and vertical lines. All have the power of bringing both lines into focusl 
at once. Of nine observers, four have the best resolving power for horizontal lines, 
one for vertical, and four equally good for either; the better resolving power 
generally coinciding with the shorter focus. 
Theoretically, at double the distance double the space between lines should he 
required to admit of their resolution. In most instances it was found, by experi- 
ment, that less than double the space was required at double the distance—a fact 
resulting probably from the aberrations varying with the adjustment, as the results 
of the observations were only taken account of when the observer believed the focal 
adjustment to be accurate. It was found that in using two eyes we generally have 
results corresponding with the powers of our best eye. Different observers describe 
the appearance of a line out of focus in various manners, as follows :—a faint band 
but with clear edges; a line with misty edges; a band consisting of two or more 
misty lines, sometimes nearly clear, and not constant in number—the motions of 
the eyelid will frequently alter the number. 
The evidence regarding the variation of sight before and after breakfast was con- 
flicting ; but all agreed that bodily or mental fatigue very much impaired both 
the resolving and adjusting powers. 
On a new kind of Miniature possessing apparent Solidity by means of a 
Combination of Prisms. By H. Swan. 
By this invention is obtained a miniature representation of the human form or 
other objects possessing the appearance of perfect solidity, the image being appa- 
rently imbedded in the thickness of a small enclosed block of glass or crystal, 
thereby defining form and expression with a degree of accuracy unattainable in a 
flat portrait. This is effected by a new application of the principles of binocular 
yision employed in the ordinary stereoscope. A stereoscopic pair of transparent pic- 
tures (taken at an angle suitable for the effect intended) are produced by the ordinary 
photographic means. To effect the combination of these, the block of glass or qua- 
drangular prism, in the interior of which the solid image is to appear, is composed 
of two rectangular prisms ground to an angle of about 39° or 40°. These are placed 
together so as to form one solid quadrangular prism, divided lengthwise by a thin 
film of air. If one of the pictures be now placed at the back of this combination, 
and the other picture at the side, on attempting to look through the combination 
the two images will be superposed on each other (forming one solid image, appa- 
rently imbedded in the crystal), all the rays which fall on one side of a line per- 
endicular to the surface of the prism next the eye sufferine total reflexion at the 
inner oblique surface of that prism, while nearly all those rays which fall on the 
other side of this line will be transmitted, unaltered in direction, through the body 
of the combination. Thus, one of the eyes perceives only the object at the back of 
the prisms, while to the other eye the picture at the side is alone visible, and that 
lying apparently at the back also, producing the perfect appearance of solidity. It 
is evident that, to produce these results, care must be taken not only that the 
pictures are not misplaced so as to produce the pseudoscopic effect, but also that 
the picture which suffers reflexion shall be reverted to compensate for the rever- 
sion occasioned in reflexion. 
On the Elasticity of the Vapour of Sulphuric Acid, By T. Tarz*, 
The author gave general formule (derived from the results of his experiments) 
expressing the law connecting the pressure and temperature of the vapours of sul- 
phuric acid diluted with different equivalents of water. 
* The paper is printed in Phil. Mag. Dec, 1863. 
1863. 2 
