164 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



out costliness) in the mechanical and optical character of the micro- 

 scopes commended and approved. 



A low-priced student's microscope of good workmanship and 

 perfect design could easily be devised if the demand for it arose. 

 Indeed, quite recently a certain class of students' microscopes have 

 been improved greatly ; this has been a concomitant of the science of 

 bacteriology, which has compelled the use of the sub-stage condenser. 

 We have said enough of the value of this instrument in a succeeding 

 chapter, but until recent years histologists did not use it because it 

 was not used in Germany or with German instruments ! Its present 

 use, nevertheless, has had the effect of improving the definition 

 obtained by the objectives used by students generally. Some who 

 perceive this, endeavour to attribute it to the improvement effected 

 in modern objectives, but this is not the case ; the objectives in 

 many cases are not even new, and until the introduction of the Jena 

 glass l the ordinary students' objectives were not really so good as 

 the English objectives of forty-five years ago. But it could easily 

 be shown that one of these early objectives, used as it always WM> 

 with a condenser, would surpass in the sharpness of its definition tilt- 

 majority of those now supplied to 'students ' with Continental models. 



But it must not be supposed that it is only the Continental 

 model that is deformed by the adoption of this radical error in the 

 ' fine adjustment ' with which we are dealing. Even during the last 

 twenty years it has been applied to some of the most imposing and 

 expensive instruments made in England and America on what i> 

 known as the ' Lister ' model. This model has one supreme virtue, 

 in the possession of a solid limb. This may take many distinct 

 forms, but it is sufficiently represented in fig. 125, where it will be 

 seen that the ' limb,' which is swung between the pillars, and which 

 carries the body-tubes and the fine adjustment, is in one solid piece. 

 If nothing were sacrificed this would be a boon. Formerly, this 

 in >dt'l was supplied with a fine adjustment which only moved the 

 nose-piece, but on a principle which we shall see was w r rong, and 

 from its imperfections it was abandoned, and the solid Lister arm was 

 rtif. and the whole body and its coarse adjustment was pivoted on the 

 lever of the fine adjustment. Thus its normal virtue (a solid limb) was 

 sacrificed, and a ' fine adjustment,' doomed to failure, was given to it. 



A complex roller, a wedge, and a differential screw have in turn 

 been since employed to redeem this instrument from the failure that 

 had overtaken it. Partially, or completely, each has failed. The 

 differential screw certainly conies theoretically nearest to success 

 with this form of instrument. But at the outset this is the case 

 only where it wholly abandons the lifting and lowering of the body- 

 tube <fec. by the action of a * fine adjustment,' and its motion is only 

 brought into operation upon the equivalent of a nose-piece. 



The form of differential sci'ew brought into practical operation 

 by the Rev. J. Campbell, of Fetlar, Shetland, was adopted by Swift 

 and Son in 1891, but had been exhibited in a stand made by Baker 

 in the year 1886 at the Quekett Micro. Club. 2 Its object is to sup- 



1 Vide Chapter I. 



' Journ. Q.M.C. ser. 2, vol. ii. pp. 283 and 287 (1886). 



