WITH THE MICROSCOPE. 9 



with a more delicate motion for altering the focus when high 

 powers are employed. The details of the arrangement of the fine 

 adjustment are different in various instruments. The movement of 

 Mr. Ladd's chain is so regular and delicate as to supersede the 

 necessity of a fine adjustment. 



12. The Body of the Microscope. The instrument should be 

 perfectly steady, whether the body be inclined or arranged in a 

 vertical position ; and not the slightest lateral movement or vibration 

 should be communicated to the body of the microscope when the 

 focus is altered by turning either of the adjustment screws. The 

 base or foot should be sufficiently heavy to give steadiness, and 

 should touch the ground in three places only, or the body should be 

 fixed upon three feet. 



The body ought to be provided with a joint by which it may be 

 inclined or placed in a horizontal position, which is required when 

 drawings are made with the camera, or when objects are measured 

 by the aid of the instrument. Another advantage gained by this 

 moveable joint is that the muscles of the observer's neck do not 

 become so tired when the body of the microscope is inclined as 

 when the head has to be bent, for several hours at a time, over an 

 instrument standing upright. The larger the microscope may be, 

 the more necessary is this joint for the comfort of the observer ; and 

 as it in no way impairs the steadiness of the instrument, and only 

 adds a few shillings to the expense, I recommend every one, in 

 choosing a microscope, to select an instrument which may be placed 

 in a vertical, inclined, or horizontal position. 



13. The stage should be at least three inches in length by two 

 and a half in width, and there should be a distance of at least 

 an inch and a half from the centre of the opening in the stage over 

 which the slide is placed, to the upright pillar a, fig. 9, pi. I. The 

 stages of the microscopes of Nachet, Oberhauser, and some other 

 foreign makers are too contracted for convenience. 



14. Hiaphragiii. Beneath the stage a circular diaphragm plate with 

 holes in it of several different sizes, should be so arranged that it can 

 be made to revolve without difficulty and any hole brought under the 

 object; a catch is of great advantage in placing the hole in the 

 centre of the field, fig. 9. Various arrangements have been adopted 

 for altering the size of the aperture in the diaphragm instead of 

 having a revolving plate with holes of different sizes. One of the 

 most ingenious is that devised by Mr. B. Kincaid (Mic. Journal, 

 July, 1866, p. 75). This is made of a short piece of thin India-rubber 

 tube, the two ends of which, fixed to brass rings, are made to revolve 

 in opposite directions so that the central part becomes contracted. 



