ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 801 



is about the fiftieth of an inch. The pinions are made to work on the 

 same axis for convenience of being controlled by one hand. 



The top plate has an improved spring arrangement (largely 

 adopted in America), for securing the slide ; upon unscrewing the 

 milled head on the left hand, the spring may either be turned aside or 

 removed altogether, when the stage will be free to carry a trough or 

 other large object. 



The usual graduated " finders " are added, but instead of the 

 ordinary pointers, verniers, reading to yvo ^^ ^^ inch, are applied. 



The circular stage rotates within a broad fixed ring. This ring is 

 preferably made of phosphor-bronze, and is graduated round the whole 

 circle, and the readings are taken by verniers. For petrological or 

 other purposes requiring exact angular measurements, the verniers 

 will be found of service. In the stage figured the range of the 

 rectangular motions has been limited to an inch in either direction, 

 but when requisite this can be largely increased. 



The total thickness of the stage is only about | inch. Actual use is 

 necessary to determine whether the endeavour to get extreme thinness 

 has or has not been carried too far so as to give liability to flexure. 



" Butterfield " Gauge of Screw for Objectives. — Frequent refer- 

 ences have recently been made in the American journals to a gauge of 

 screw for objectives of much larger diameter than that of the " Society " 

 screw. It is known as the " Butter- 

 field " gauge, and is shown in Fig. 47 Fig. 47. 

 (actual size) in the form of an adapter 

 to screw directly into the microscope- 

 tube ; the opening beneath is of the 

 usual " Society " gauge. The purpose 

 of the new gauge is to permit the ex- 

 tension of the apertures of low powers 

 by the utilization of much larger dia- 

 meters of back lenses than can be utilized with the smaller gauge — • 

 the latter would, in fact, act as a diaphragm to the new objectives, 

 cutting off a portion of the effective aperture. 



Homogeneous-immersion Objective with extra Front Lenses.— 

 Messrs. Powell and Lealand have completed a -^ having two extra 

 front lenses on the plan noted in vol. iii. p. 1050. The maximum 

 numerical aperture is 1*43 (= 140° in crown glass of mean index 

 1 • 525), obtained by a front lens several degrees greater than a hemi- 

 sj)here, mounted on a plate of glass "003 inch in thickness, which 

 is itself mounted in the usual metal-work by the zone projecting 

 beyond the circumference of the lens. 



With this front lens the focal distance from the exposed face of 

 the plate on which the lens is mounted is "007 inch. 



A second front, nearly a hemisphere, is mounted in the usual way 

 by a burred edge of metal covering the extreme margin of the lens. 

 This front gives a numerical aperture of 1'28 (= 115° in glass), and 

 the focal distance is then '016 inch. 



The third front provides a numerical aperture of 1*0 ( = 82° in 



Ser. 2.— Vol. I. Y 



