THE MICROSCOPE. 
for the purpose of changing the angles. In general, and more 
especially when high powers are used, the object-glasses are so 
close to the stage, that they cannot be conveniently unscrewed 
and changed without either removing the object-tube from the 
stage, or the latter from the former. If, however, the body be 
turned horizontally upon the centre a through a few degrees, the 
object-tube will be removed from over the stage, and the lenses 
can easily be changed. 
This method may also be practised in the positions shown in 
figs. 38 and 40, but it is more convenient to turn the rectangular 
piece yx upon the bayonet-joint, as directed above. 
Another advantage which attends this horizontal motion of the 
body round the centre a, is, that it enables the observer to direct 
fhe object-glass successively on different points of an object, the 
whole of which is not included in the field of view. This, how¬ 
ever, can only be practised where low magnifiers are used. 
To place this microscope in any desired inclined position, it is 
only necessary to place the body, as represented in fig. 38, and 
then taking out the pin a, fig. 37, to turn the bar leg together 
with the body e into the desired inclination. 
boss’s impeoved miceoscope. 
81. Mr. Ross holds a place in the foremost rank of philosophical 
artists, and deservedly enjoys an European celebrity. 
To his labours, perseverance, and genius, much of the perfection 
attained in the construction of object-lenses is due. The 
adjusting object-piece, already described, is one of his recent 
inventions (19). 
In the progressive improvement which the microscope has 
undergone in his hands, the stand and the mounting, with the 
provisions for the arrangement of the accessories, have of course 
been more or less modified from time to time, and are at present 
varied according to the price of the instrument, and the purposes 
of the observer. 
82. We shall here give a short description of the most recent 
form given by him to his best instruments. 
Upon a tripod, 1, 1, fig. 41, are erected two upright pieces, 2, 2, 
strengthened by inside buttresses, 3. These uprights support an 
horizontal axis, 4, which passes nearly through the centre of gravity 
of the instrument, and upon which it turns, so that the axis of the 
body may be placed in any direction, vertical, horizontal, or 
oblique. The rectangular bar, 5, having a rack at the back, is 
moved in the box, 6, by the pinion, 7. The body, 8, is inserted 
in a ring at the end of the arm, 9, which latter is fixed upon a 
pin at the end of the rod, 5, upon which it turns, so as to remove 
78 
