131 



Two -Speed Fine Adjustments. 



By a. Ashe. 



{Read October I8th, 1901.) 



The introduction of wide-angled lenses has brought about, as an 

 inevitable consequence, a great improvement in the construction 

 of fine adjustments. 



These improvements are still being made by leading opticians, 

 and quite recently both Messrs. Zeiss and Reichert have brought 

 out very novel forms — suited, of course, to the Continental type 

 of instrument. 



Up to the present, these firms, as well as others, have, with a 

 single exception to be referred to later, apparently worked with 

 only one object in view, and that is, the securing of an almost 

 imperceptible movement of the focal point when the milled head 

 is moved. Of course I do not include smoothness of motion, for 

 without this as a foundation all labour spent in improving the 

 design, or extending the usefulness of the adjustment, is thrown 

 awav. 



Now, while it is true that oil-immersion objectives, when used, 

 as they should be, in conjunction with wide-angled condensers, 

 demand the utmost delicacy of motion obtainable, with regard 

 to lenses of somewhat lower power and aperture this extreme 

 slowness is not required to anything like the same degree ; and 

 when, as is often the case, the object under observation is of 

 sensible thickness, covers a large area on the slide, and possesses 

 numerous points at various depths, all of which it is necessary 

 to rapidly examine, then the value of a comparatively quick fine 

 adjustment will be reahsed. 



Some will say that recourse can be had to the rack and 

 pinion under such cii^cumstances ; but the rack, though quite 

 capable of focussing a X2"^^- objective upon any particular point, 

 is not adapted to the rapid following of focal planes and the 

 tracing of their inter-relationships. 



On the other hand, an ultra-slow adjustment involves in some 

 work an enormous waste of time. This is especially noticeable in 



