1 64 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



nut costliness) in tin- mechanical and optical character of the micro- 

 scopes commended and approved. 



A lo\\ -priced student's microscope of good workmanship and 

 |,ei-fect design could easily lie devised if the demand for it arose. 

 Indeed, .piite recently a certain class of students' microscopes have 

 l>een improved greatly : this has been a concomitant of the science of 

 haeterio|o-\ . \\ Inch lias compelled the use of the sub- stage condenser. 

 \\'e lia\e said enough of the value of this instrument in a succeeding 

 diaptei-. l.ut. until recent years histologists did not use it because it 

 was not used iii (iermany or with German instruments ! Its present 

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

 obtained by t he 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 

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

 glass 1 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 was 

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

 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 dining the last 

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

 expensive instruments made in England and America on what is 

 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 tine adjustment, is in one solid piece. 

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

 m idel was .supplied with a fine adjustment which only moved the 

 nose-piece, but on a principle which we shall see was wrong, and 

 from its impel- feet ions il was abandoned, and the solid Lister arm was 

 cut, and the whole body and its coarse adjustment was pivoted on the 

 lever of the line adjust ineiil . Thus its normal virtue (a solid limb) was 

 sacrificed, and a ' line adjust nient .' 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. Part ially. or completely, each has failed. The 

 differentia] scre'w certainly comes theoretically nearest to success 

 with this form of instrument. l!ut at the outset this is the case 

 onl\ where it wholly abandons the lifting and lowering of the body- 

 bube &c. b\ the action of a ' line adjust meiit .' a ml its motion is only 

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



Tin- form <>f il[ij'< r, nt'ni! screw />,-//,//// ,',/ti, nractical operation 



. Campbell, of Fetlar, Shetland, was adopted by Swift 



v.n iii IX'.H, but had been exhibited in a stand made by Baker 



year 1886 at the^uekett Micro. Club.- Its object is to sup- 



apter I. 

 n (,>.M.C. ser. '2. vol. ii. pi>. -2s:; :l ml 287 (1886). 



