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 <?, is, that it enables the observer to direct 

 the 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 D F a together 

 with the body R into the desired inclination. 



ROSS'S IMPROVED MICROSCOPE. 



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, o, 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, o, upon which it turns, so as to remove 

 78 



