100 CONSTRUCTION OF THE MICROSCOPE. 



affords the only means of making the " coarse" adjustment ( 25, 

 II) ; but a rack-and-pinion movement may be introduced at a 

 trifling additional cost. The "fine" adjustment is made by a 

 milled head in front of the lower end of the body, which acts 

 directly upon a tube sliding within it that carries the objectives. 

 This instrument is particularly adapted, by the roominess of its 

 stage, for the examination of pathological specimens; and, when 

 the body is provided with a rack movement, it forms an unex- 

 ceptionable microscope for general purposes, and may even be fur- 

 nished with a movable stage, achromatic condenser, polarizing 

 apparatus, &c. ! 



33. Nachet's Microscope. Until a comparatively recent period, 

 all save the most elaborate and expensive forms of Compound 

 Microscope constructed by Continental opticians, were adapted 

 for use in the vertical position only. M. Nachet, however, has 

 now so modified his ordinary pattern, that the instrument may 

 be inclined (like the preceding) at any angle ; and he has thus 

 rendered it a very convenient, as well as a cheap and portable Mi- 

 croscope. The basis consists of a somewhat oval foot, with a 

 single pillar rising from a little behind its centre ; and at the top 

 of this pillar is a " cradle-joint," which supports the stage and 

 the upright stem that carries the body. The transverse arm, how- 

 ever, is attached, not directly to the summit of this stem, but to 

 a tube which slides over it ; and this tube can be raised or 

 lowered by turning the milled head at its summit (which acts 

 upon a screw that enters the stem), whereby a "fine" adjustment 

 is obtained, that acts through the transverse arm upon the body 

 which it carries. The " coarse" adjustment is effected, as in the 

 preceding case, by sliding the body through an outer tube which 

 grasps it ; the latter being fixed into the transverse arm. The 

 mode in which the object is supported upon the stage, when this 

 is inclined, is very simple and ingenious, and is in some repects 

 preferable to the sliding-ledge generally used by English makers. 

 Near each side of the stage is seen a somewhat elastic strip or 

 tongue of sheet brass, the front extremity of which is free, but 

 which is attached at its hinder end to a pin that passes through 

 a hole in the stage, in which it works very easily. This pin is 

 prolonged for about f ths inch beneath the stage, and then termi- 

 nates in a broad flat head ; and it is surrounded by a slender 

 spiral spring, which, bearing at its two ends against the under 

 side of the stage and the head of the pin, tends to depress the 

 latter, and thus to bring the brass tongue into close apposition 

 with the stage when nothing intervenes, and to bind down any- 

 thing that may be placed between them. In making use of 

 this little apparatus, it is most convenient to employ both hands, 

 in such a manner that the thumb and forefinger of each shall 



1 The cost of this instrument essentially depends upon the number and magnifying 

 power of the objectives supplied with it; it is usually provided, however, with a 1-inch 

 and -inch; and is then sold (without the rack movement) at 6 I Os. This sum, how- 

 ever, does not include either a case or any accessory apparatus. 



