56 On Obtaining a Large Field of View, &c. [Sess. 
draw-tube of a microscope. This arrangement caused a con- 
siderable loss of light. It was devised long before photog- 
raphy was used in connection with the microscope. The only 
other method of increasing the field of view was by means of 
a specially constructed object-glass, first introduced by Zeiss, of 
Jena. This consisted of an achromatic negative lens similar 
to a Barlow at one end of the objective mount, while at the 
other end there was a positive achromatic lens. These two 
separate lenses were so mounted that the distance between 
them could be made variable, and thus get different powers 
according to the position in which they were placed. One 
by Wray gives powers equivalent to a lens of from 4 to 6 
inches focus. It is constructed to be used on a 10-inch 
tube. It is a very excellent objective. 
Some time ago, when constructing a small microscope with 
a Jackson arm, the draw-tube was made out of a short piece 
of tube which had been used for a different purpose. It was 
only 44 inches long. The Jackson arm was too short to 
admit of its carrying a 3-inch objective in the ordinary way, 
and this objective was accordingly screwed into the lower end 
of the draw-tube. On pulling out this tube it was in this 
way easy to get the necessary distance to focus the objective. 
It was found that with the low power eyepiece used a very 
large field of view was obtained. A crown piece, 14 inch in 
diameter, was easily seen in the field, while the definition was 
everything the most fastidious could desire. The eyepiece used 
in combination with the 3-inch objective was a No. 0, by Leitz 
of Wetzlar. It is built on the same principle as the Kellner 
orthoscopic—that is to say, the focus of the eye lens reaches 
to the field lens. The eyepiece tube has no diaphragm, as the 
cell in which the field lens is mounted forms the diaphragm. 
The only difference between this eyepiece and an orthoscopic 
is this—the Leitz eyepiece has a plano-convex eye lens, while 
the orthoscopic has a concavo-convex one. The Leitz No. 0 
has an extremely flat field and low power, its equivalent focus 
being about 2 inches. No other eyepiece is so well adapted 
for obtaining the large field in the way indicated. With the 
above combination of 3-inch object-glass and No. 0 eyepiece, 
smaller fields of view may be obtained by simply pulling out 
the eyepiece a short distance, say from half an inch to an inch, 
