174 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



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

 within the bar, and actuated by a micrometer screw with a milled 

 head at one end, the fulcrum being at the other, and the nose- 

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

 angled objectives ; but there were two faults belonging to it : one 

 was 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 had a bar movement : but the bar being of relatively great 

 length, he employed a lever of the first order, the micrometer-screw 

 being at one end, 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 -g-J-^th 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 importation of vibration to the body ; and. as tin ' nose- 

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

 this 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 position and character will be seen on the right-hand 

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

 what we now 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 steadiness of movement does not belong to it. It is only that it 

 was concurrent with the belief in * low angles,' and consequent * pene- 

 tration ' in objectives (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 we learn that there are three types of micro- 

 scope models for which a suitable fine adjustment has been found. 



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

 lever is the perfect method. 



ii. The Lister model, for which Swift's vertical lever and 

 Watson's long horizontal lever are the best forms known. 



iii. The Continental model, for which Campbell's differential 

 screw is the most smooth and delicate device yet suggested, unless 

 we take into consideration the beautiful lever fine adjustment of 

 Reichert. 



The full value of delicacy in the fine adjustment can of course 

 only be fully appreciated by the expert. A tolerable speed may be 

 permitted in this adjustment when uncritical images with small 

 illuminating cones are used, because objectives so used are far less 



