AND THE SIDE SCREW FINE ADJUSTMENT. 97 



Powell kindly showed me one of these microscopes, but it had 

 escaped my memory. The coarse adjustment was by rack and 

 pinion ; this was not attached to the limb by a slide, but by a 

 kind of cradle. This cradle was pressed down by a spring on 

 to a horizontal cone, which was moved by a horizontal fine- 

 adjustment screw, which had a milled head on each side of the 

 limb. 



The importance of this model should be recognised by every one 

 who uses a microscope, for not only was it the first microscope 

 to have a side screw, but also it was the first instrument in 

 which we find the limb attached to the foot on two upright 

 pillars. This double support to the joint (now almost universally 

 used) was the invention of George Jackson (President R.M.S. 

 1852-3). Before this all microscopes that were capable of being 

 inclined were attached to the foot by a single upright post and a 

 compass joint.* Powell attached the two pillars to a flat tripod 

 base by a swivel so that the base could be placed in such a 

 position as to give the greatest amount of stability however 

 much the body might be inclined (some makers in copying this 

 arrangement graduated this arc of rotation !). 



Ross copied this kind of joint in the model he brought out in 

 1843, but substituted two parallel flat plates for the two pillars ; 

 but Messrs. Smith and Beck adopted the two-pillar form in their 

 1846 model. 



This microscope of Powell's had a Turrell stage, a micrometer 

 stage, an achromatic condenser, Nicol polarising and analysing 

 prisms ; so it was in its day an instrument of a very advanced 

 type. In 1843 Powell & Lealand discarded the two pillars for 

 the gipsy tripod, which is the best form of foot ever designed.f 



Coming now to modern times, horizontal fine adjustments may 

 be placed in two groups, viz. (a) those with continuous motion 

 and (b) those without. The drawback which those of the first 

 kind possess is that the user does not know whether he is 

 focusing up or down ; and the drawback which all the second 

 kind, excepting the Berger, have is that of damage and injury to 

 the delicate moving parts when they butt up against a stop. 

 The Berger avoids all risk of damage from this source by causing 



* Some ancient non-achromatic microscopes had ball and socket joints, 

 but those early forms are not now under discussion, 

 t For fig. see Journal R.M.S., 1900, p. 289, fig. 79 

 Journ. Q. M. C, Series II. No. 72 7 



