165 



XXIII. — On a Lever Movement to the Stage of the Microscope. 

 By Alfred White, Esq., F.R.S., &c. 



Read November 15, 1843. 



The lever movement to the stage of the microscope having been 

 very recently under the consideration of the Society, I am induced to 

 lay before you a description of the plan I have used during the last 

 three years, and find it much more convenient than the rack and pin- 

 ion, or the screw. 



It does not at all depend upon a particular construction of the 

 guides to the stage, but only to the motion of its upper plate, or the 

 one upon which the object to be viewed is placed; therefore to a stage 

 with three, or to one with two plates, this plan is equally applicable. 



It consists of a lever D, E (Plate xx.), to the shorter arm of which 

 the ball B is firmly screwed, and moves in a socket formed by the 

 upper plate of the stage A, and the cap C, which is a brass plate se- 

 cured by two screws. This lever passes through the perforated ball, 

 F, which moves in a socket formed by the arm G, which is attached 

 to some immoveable part of the microscope, and the cap H, formed 

 and secured as C. 



The lever I have used is about 5 inches long, and has its longer 

 arm E = 3, and the shorter, D, = 1, when in the position represent- 

 ed in fig. 1, (Plate xx.). But this proportion, when in use, is not 

 constant, as will be seen by fig. 2; for the longer arm of the lever hav- 

 ing been elevated and the shorter one depressed, and the ball B hav- 

 ing travelled in a straight line to 5, instead of a portion of a circle, it 

 will be found that the shorter arm has increased by a quantity equal 

 to the space between 1/and 2/, and for this reason the ball F is per- 

 forated to enable that part of the lever to move easily, though without 

 shake. 



As the end of the lever to which the hand is applied moves in all 

 cases in an opposite direction to the ball B ; and as the compound 

 microscope inverts the image of the object viewed; it follows that the 

 object will appear to move in the direction of the hand : and as for 

 each position of the ball B, there is but one in which the other end of 

 the lever can be placed, if a certain portion of the object is seen at 

 any time, and it is required to bring the same part into the field again, 

 this will be done by placing the hand in the original position. 



