Aprii., 191: 



KNOWLEDGE. 



155 



the rate of motion. 

 The prismatic bar is a 

 most secure and accur- 

 ate guide, and when it 

 is used the. action of 

 the micrometer screw 

 is generally direct, and 

 raises or lowers the 

 sleeve which carries 

 both the arm and tube 

 •f the microscope. 

 Dove-tailed slides are 

 usually placed immed- 

 iitely behind the coarse 

 adjustment and carry 

 IT and the tube 



FlGURK 179. Lever Fine 

 .Adjustment. 



Figure 17S. .Ariston Fine 

 .Adjustment. 



only, with the advantage that the arm 



may be lengthened to give greater 



stage -room without adding to the 



weight borne by the fine adjustment. 



A direct-acting micrometer screw i.~ 



under these conditions inconvenient. 



and some intermediate mechanism i> 



usual, generally a lever, but a few 



maimers use a cam or some adaptn- 



tion of the principle of a wedge. 



There are many different varieties of 



these broad types, and the methods 



by which the screw or lever, and so 



on, are utilised, are more or less 



special to individual makers. The six examples given are 



selected as showing a fairly wide range. 



The Direct-acting Micrometer Screw or Prism.-^- 

 Tic B.AR Fixe Adjustment. Figure 177 shows the form 

 adopted by Zeiss. The arm (A) and tube of the microscope 

 are carried on a sleeve (B) which slides upon a vertical tri- 

 angular bar (C). A nut (E) is screwed on to the top of (C) 

 and a spring presses against it and against (Hi. The hardened 

 steel point of the micrometer screw bears on the similarly 

 hardened centre of (HI and works through (Fl which is 

 screwed to the sleeve. When the micrometer screw is turned 

 downwards the tube travels upwards and the spring is 

 compressed. The latter in its turn causes the downward 

 motion as the screw is turned up. This then is a safety fine 

 adjustment, as the objective cannot be screwed by it down 

 on to the cover glass. Most prismatic bar fine adjustments 

 :ire liable to damage if the microscope is lifted by the 

 moveable sleeve, but in this case the danger to the micrometer 

 screw is minimised by fi.xing it to the tube carrier. If the 

 microscope is lifted by the latter the screw is lifted too, while 

 the lower part of the instrument drops until stopped by the 

 counter-nut screw on top of the prismatic bar. 



The .\riston Fine Adjustment. — The fineness of 

 motion imparted by a direct-acting micrometer screw obviously 

 depends on its pitch, which is generally about one-fiftieth of 

 an inch ( -5 mm. I. and one rotation of the milled heads raises or 

 lowers the microscope tube through that distance. This is not a 

 ver\- slow motion, though adequate for most purposes, and several 

 methods have been adopted to make the movement slower. 

 It is uot advisable to use much finer screws, as they are very 



liable to damage. Messrs. Swift reduce the speed to one-fifth 

 by interposing two levers, while retaining the general character- 

 istics of the prismatic bar type as shown in the Ariston fine 

 adjustment, (See Figure 17K.) This and most other prismatic 

 bar fine adjustments differ from that of Zeiss in that the 

 pressure of the spring raises the microscope tube, while the 

 action of the screw presses it downwards ; that is, they are not 

 generally of a safety type. On the other hand, the .\riston 

 has the great advantage over most in that the moveable sleeve 

 carrying the limb has a protecting cover screwed over it, by 

 which the microscope may be lifted without damage to the 

 mechanism. The Spencer Lens Company give similar pro- 

 tection by fitting a small guard below the limb. 



Lever Fine Adjustment. — Figure 179 shows a simple 

 but very efficient way of using the lever in fine adjustments, a 

 method that, with variations, in detail 

 is very largely employed on English 

 and .American stands. It has the follow- 

 ing points of interest. By placing the ful- 

 crum of the lever in suitable positions, 

 any desired ratio can be got between 

 the upward motion of the end of the lever 

 carrying the tube and the downward move- 

 aent of the other end caused by the screw. 

 A very fine motion can consequently be 

 obtained from an ordinary micrometer 

 screw. For instance, Watson and Beck 

 provide a movement of one three-hundredth 

 of an inch with a screw of one-sixtieth of 

 an inch pitch. In the form adopted by 

 Baker the screw is of one-seventieth of 

 an inch pitch and the arms of the lever 

 are as three is to one, so that one rota- 

 tion of the milled head raises the tube one 

 two-hundred and tenth of an inch. It is 

 a safety adjustment, as the downward 

 motion is caused by the weight of the 

 tube and the spring, which also keeps the 

 lever always up to its work against the 

 screw. The bearing points and surfaces 

 are, as in all the adjustments, of hardened 

 steel. -And the limb can be used as a handle 

 without any danger to the mechanism. 



Figure ISO. Baker's New Lever Fine .Adjustment. 



