TRANSACTIONS 
OF THE 
MICROSCOPICAL SOCIETY 
OP 
LONDON. 
On the Application of Binocular Vision to the Microscope. Bj 
F. H. Wenham. (Read May 25, 1853.) 
On viewing objects by the unassisted eyesight there are two 
conditions which enable us to appreciate or judge their various 
distances. Firstly, the object is observed by each eye from 
a separate point of view, and the consequent difference of 
outline, light, and shade between the images formed on each 
retina allows us to form an accurate idea of their various 
sizes and positions. The angle of stereoscopic vision has been 
stated somewhat definitely to be about 18 degrees, but this 
must be subject to considerable variations, as whether the ob- 
server is long or short-sighted, the difference of distance 
between the eyes, and also the bulk, form, and position of the 
object. I may state that I have obtained a very good perspec- 
tive of minute objects when the angle of vision has exceeded 
50 degrees. 
If we perforate a card with a pin, and examine the articles 
in a room illuminated by candle-light, with one eye looking 
through this aperture, we shall be able to judge of distance 
only from the relative intensity with which the objects are 
illuminated, the nearest receiving and giving off the greatest 
quantity of light, and the farthest being in comparative darkness. 
I make these preliminary observations because, in viewing 
the greatest portion of objects under the microscope, the con- 
ditions here referred to, which give us the faculty of judging 
of bulk and distance, do not exist in the same degree, if at 
all. In the first place, in viewing an object, as a transparency, 
with a single lens of short focus, we see it under such circum- 
stances as seldom happen to such surrounding objects as come 
under our daily observation, and in the illumination of trans- 
parent objects by direct transmitted light, the effect is the 
reverse of that which is necessary for us to judge of distance 
VOL. II. b 
