ISfi 



KNOWIJIH-,!-. 



Al'Kil., 1912. 



Baker's Nkw Lever Fink 

 Adjustment. — Figure 180 is from 

 a drawiiiK of Hakcr's new lever fine 

 adjnstinciit. aiul is n'ven as an 

 example of the met hods employed 

 for ntlli/iii»; a leviT when the milled 

 heads are placed at the sides. Beck's 

 and the Spencer Lens Company use 

 other forms of levers for the same 

 purpose. .At (.A) are the milled 

 heads; (he thread of the micrometer 

 screw (H) is liftyto the inch. Motion 

 is conveyed by a curved lever (C), 

 whose ratio is one to four, to a 

 narrow wheel (D) intended to elimin- 

 ate friction as nuich as possible. 

 The lever is curved where it touches 

 (D) in such a way as to ensure that 

 the latter carrying the tube is raised 

 an equal amount for each rotation of 

 the milled heads. A spring (F) on 

 the guide pillar (E) keeps the wheel 

 down to its work and causes the 

 downward motion. Each revolution 

 of the milled heads corresponds tu 

 a movement of -125 millimetres oi 

 one two-hundredth of an inch. All 

 slides in this and the last adjustmeni 

 are sprung and screwed. 



The position of the milled head> 

 found in this fine adjustment is ver\ 

 much in favour now, and adoptetl 

 by many makers, but it must not 

 be supposed that that indicates a 

 similarity in the working parts 

 enclosed in the limb. That of 

 Zeiss, for instance, is actuated by 

 a micrometer screw (see Figure 181). 



Bergek's Micrometer Screw 

 Fi.NE Adjustment. — (Figure 181). 

 The milled heads actuate a worm 

 which in its turn rotates a cogwheel 

 attached to the micrometer screw. 

 The latter works in a socket firmly 

 fixed to the sliding bar, so that if 

 the milled head is turned to the 

 left the tube will be raised. When 

 turned to the right the socket is 

 brought downwards on to the screw 

 by the spring and tube weight 

 only and disengages from the screw- 

 should the objective touch the cover 

 glass. The guiding cogwheel to the 

 left moves up or down as the 

 milled heads are turned until it 

 reaches the casing and so acts as a 

 stop to the maximum and minimum 

 movement of the adjustment. One 

 rotation of the milled ht^ads gives a 

 movement of one twohundred-and- 

 fiftieth of an inch or 0- 1 millimetrc•^. 



Leitz' New Fine Adjustment 

 (Figure 182) is illustrated as an 

 example of one of the fine adjust- 

 ments in which mechanical arrange- 

 ments other than a screw and lever 

 are employed. The lateral milled 

 heads convey motion to a wheel td) 

 by worm gearing. This wheel carries 

 a heart-shaped eccentric cam (f),aiid 



on this a steel roller is pressed by the weight of the tube 

 assisted by a spring. .As the cam is turned the roller which 

 revolves upon it is lifted upwards carrying the microscope 

 tube with it, or falls by the weight of the latter and the spring. 



I'lGl'KE ISl. 



Berger Micrometer Screw Fine .Adjustment 



1-Tguke 182. 

 Leitz' New F"'ine .Adjustment. 



One revolution of the milled head 

 corresponds to a rise or fall of • 1 

 millimetre (jtn-inch). Examination 

 of the figure will show that instead 

 of a limit to the movement of the 

 milled heads in both directions, as 

 there is with most fine adjustments, 

 the motion is continuous and a 

 further rotation when the tube is 

 raised or lowered by the cam to its 

 greatest extent instantly reverses the 

 motion. The rise and fall due to 

 the rotation of the cam is three 

 millimetres, so that reversal need 

 very seldom occur. Continuous fine 

 adjustments have the advantage that 

 there is no fear of damage due to 

 the arrival of the movement at its 

 upper or lower limit. But the 

 uncertainty as to whether one is 

 focussing up or down is occasionally 

 a drawback, although a glance at the 

 indicator on the sliding bar and limb 

 serves to show the direction of 

 movement, and as the milled heads 

 can be made to work in the same 

 direction as those of Ihe coarse 

 adjustment at any time the difficulty 

 should seldom arise. 



The milled heads of any fine 

 adjustment can be had graduated 

 in fifty or one hundred divisions, 

 so that the amount of motion given 

 can be determined when required 

 for purposes of measurement. 



The testing and selection of a 

 fine adjustment is almost as im- 

 portant as the choice of objectives ; 

 for unless the former is satisfactory 

 it is impossible to use the lenses 

 to their best advantage. The work 

 put into modern microscopes is so 

 uniform in quality that the efliciency 

 of any adjustment fitted by a maker 

 of repute may be relied on. One 

 may be slower in motion than 

 another, but it is not fair to say on 

 that account that it is better, unless 

 the use to which the instrument 

 will be put is known. .A movement 

 of one-fiftieth or one-sixtieth of an 

 inch for each rotation of the milled 

 heads is quite adequate for low and 

 medium powers, and in many cases 

 preferable to a slower one ;is 

 more durable and tending to save 

 time in laboratory work. But for 

 high powers and particularly lor 

 photo-micrography it is ditficult to 

 ;:< t too slow a fine adjustment. 

 I ' advantages of a fast and 

 motion are often combined 

 the same instrument by fixing 

 a spindle of small diameter to the 

 centre of the milled head. It may 

 be rapidly rotated when a faster 

 movement is desired. Secondary 

 levers are sometimes added to reduce 

 the speed of a fast type, as in the 

 .Ariston. or the M.ales.\\"atson. which 

 is provided with two milled heads. 

 Becks use two concenlric micro- 

 niler screws of difterent pitch to actuate their lever fine 

 nljustment, giving movements of one-si.xtieth and one-three- 



hundredth of an inch respectively. 

 Safety types have a distinct 



advantage over other 



