102 Mr. Dollond’s account of a 
also of taking intermediate angles between zero and the 
greatest separation of the images, without exchanging any 
part of the eye tube, it being only required to move the axis 
in which the sphere is placed. 
Thirdly, it possesses the property of an eye tube or lens that 
is not intended for micrometrical measurements ; for when 
the axis of the crystal is parallel to the axis of the object- 
glass of the telescope, only one image will be formed, and 
that will be as distinctly formed as with any lens that does 
not possess the double refracting property. 
The eye tube is so constructed, that the plane through 
which the two images move, can be placed parallel to the 
line in the object which is to be measured ; and if this motion 
is furnished with a divided circle, it will correctly answer the 
purpose of a position micrometer. 
The value of the scale is found from the known diameter 
of any distant object, and will vary in proportion to the 
magnifying powers of the eye tube ; its value increasing in 
proportion to the increase of those magnifying powers. 
The preceding remarks appearing to be sufficient to eluci- 
date the novelty of the application, I shall now endeavour to 
render the contrivance more explicit by references to the 
plate. £See PI. IX.] 
Fig. 1. Is a section of the eye tube; and Fig. 2. a general 
view of the same ; both of the full size. 
The sphere or lens, a , fig. 1. is formed of rock crystal, 
and placed in half holes, from which is extended the axis b, b, 
with an index attached; which index registers the motion of 
the sphere, the extent of that motion being shown upon the 
divided face c, fig. 2. The sphere is so placed in the half 
