Tue Microscope. LOT 
Selections. 
EYE-PIECES AND OBJECTIVES. 
BY ED. BAUSCH, ROCHESTER, N. Y. 
[Read before the Section of Microscopy, Rochester Academy of Science.] 
HE eye-piece and objective in the microscope are called the 
optical parts, but are essentially the only parts which make 
it up. The mechanical portions which are applied to the instru- 
ment are merely the conveniences for manipulating the object, 
and adjusting for focal distance. 
The eye-piece is that portion of it which magnifies the 
image formed by the objective, and although its perfection 
depends upon its construction and accuracy, the distinctness of 
the image depends upon the perfection of the objective. All the 
various kinds of eye-pieces may be divided into two classes, the 
positive and the negative, in the former of which the focal point 
is outside of the lenses, and in front of the field lens, while in 
the latter it is internal, i. e., between the lenses. 
In both the image is inverted, and its apparent size depends 
upon the combined foci of the eye-lens and field-lens. Among 
the positive are comprised the Ramsden, the Kellner Ortho- 
scopic, the Periscopic, and others. The first is simply an ordinary 
magnifier, composed of two plano-convex lenses, and is used 
principally for micrometer work. The other two have correct- 
ing eye-lenses, and partially correct the errors in spherical 
aberration of the objective, thereby giving flatter and larger 
field. 
In our correspondence we often find that there exists a 
misconception of the word positive, arising, I suppose, from the 
fact that the eye-piece which inverts the image is called the 
negative. Many suppose that it should show ‘the image in a 
position corresponding to that of the object. Another eye- 
piece, however, is used for this purpose, which is composed of 
four lenses or systems, and is called the Erecting eye-piece. It 
is seldom used with the microscope, but it is of great value 
in the telescope, where it is called a terrestrial eye-piece. It is 
also positive. 
