174 T1IK HISTOUY AXD DEVELOPMENT OF THE MICEOSCOPE 



movement. It consisted of a lever of the second order inserted 

 // Inr. : mid actuated by a micrometer screw with a milled 

 head :ii one end. the fulcrum briny at the other, and the nose- 

 piece between them. This sewed admirably in the days of low- 

 an-led objectives; but there were two faults belonging to it: one 

 vs.,. that the tube of the nose-piece had not a sufficient length of 

 bearing and was liable to a lateral shake ; the other was that the 

 adjustment screw, being near the middle of the bar, involved tremor. 



The application of this principle in its very highest and most 

 perfectly practical form was invented by Powell. His instrument 

 also hail a liar movement: but the bar being of relatively great 

 length, he employed ,i I, r/'t- of tlie first order, the micrometer-screw 

 being at one e nd. the nose-piece at the other, and the fulcrum between 

 them. The ratio of the arms of the lever was 4:1; and the screw 

 is so arranged that a complete revolution of the milled head is equal 

 to the ., ontli of an inch. The position of the screw is immediately 

 behind the pivot on which the bar turns, and this precludes the possi- 

 bility of the impartation of vibration to the body ; and, as the nose- 

 piece tube is very long, and only bears on three points at either end. 

 t his adjustment is the steadiest, the smoothest, and the most reliable 

 for all objectives of any of the several devices which have come before 

 us during the last twenty years. In fact, this fine adjustment has 

 held an unrivalled position for the past fifty years (fig. 157). 



The fine adjustment that was employed as its rival on the earlier 

 forms of the Lister model was known as the short-side lever, and 

 it was sometimes employed in the commoner bar-movement micro- 

 scopes. Its posit ion and character will be seen on the right-hand 

 side of the body of 1 he Smith model, fig. 122. In the light of 

 what we no\\ need, we are bound to say to the intending pur- 

 chaser of a microscope. Avoid it;' it is bad alike in design and 

 construction. The screw is so placed that tremor is inevitable in 

 the body when it is touched, while the nose piece tube is so short 

 that steadine of movement does not belong to it. It is only that it 

 was concurrent \\ith t he belief in ' low angles,' and consequent ' pene- 

 tration ' in object i\ es (with which no critical work could be done). 

 that it is possible to account for the toleration for so long in num- 

 bers of English microscopes of this wholly inefficient adjustment. 



From the foregoing \\ e learn that there are three types of micro- 

 scope model.-, for \\hich a suitable line adjustment has been found. 



i. The bar movement model, for which Towell's first order of 

 level- is I he perfect met hod. 



ii. The Lister ...... lei. for which S\\ itt's vertical lever and 



Watson's Ion- hori/.oiital lever are the best forms known. 



iii. 'I'h.' Continental model, for which Campbell's differential 

 - the most smooth and delicate device vet suggested, unless 

 e into consideration the beautiful lever line adjustment of 

 tleichert. 



value of delicacy in t he line adj list meiit call of course 



appreciated by the expert. A tolerable speed may be 



adjustment when uncritical images \vith small 



used, because objectives so used are far less 



