OF ARTS AND SCIENCES. 353 



of the microscope be made of greater stability than in usual construc- 

 tions, and that the screw itself be of accurate workmanship. 



It is not material in such measurements that the zero of the scale 

 should retain a fixed position for more than a few hours together. 

 The screw is so short that it most probably is affected throughout its 

 length by the same conditions of temperature and thickening of the oil. 

 And with the micrometer screw we can apply the well-known princi- 

 ple that a bisection of a small object can be made more exactly than 

 can the distance of that oBject be estimated relative to two micrometer 

 lines contiguous ; unless, indeed, the object is placed midway between 

 two closely parallel lines, which becomes then also a case of bisection. 



In order to carry out the idea, Mr. Crouch constructed for me one 

 of his first-class microscope stands, with some modifications in it which 

 I thought necessary to insure the solidity we find in astronomical 

 instruments. A clamp is added to the axis on which the instrument 

 swings, so that it may be rigid at any inclination. The " Jackson 

 arm " contains a small clamp so that any possible play in the rack and 

 pinion can be counteracted. In the Crouch model, this arm has a 

 bearing of 17 cm. in length, and 16 mm. in width. 



Resting upon this bearing is the cradle which carries the body of 

 the microscope ; its base is 16 mm. in width, and the chord of its upper 

 circular surface is 19 mm. The body, which is constructed of brass 

 tubing, 2 mm. thick, and 36 mm. interior diameter, is soldered to this 

 cradle. The side of the cradle away from the body carries the ordi- 

 nary T rail with a smooth-working rack. The pinion is provided with 

 large heads, 5.7 cm. in diameter, and the performance is satisfactory 

 enough to readily focus a high angle sixth upon an object, without 

 resort to the fine adjustment which, in the Jackson model, unfortunately 

 alters the distance between the object-glass and the reticule in the eye- 

 piece. 



The screw and pinion moving the mechanical stage are provided 

 with large heads 3.7 cm. in diameter, for the purpose of more easily 

 re-setting upon the first line of a series in measuring the same space 

 with different parts of the micrometer screw to be hereafter mentioned. 



The ordinary triple-threaded screw for carrying the mechanical stage 

 being too coarse to allow of exact setting, Messrs. Buff & Berger have 

 replaced it for me with a screw having forty-one threads to the English 

 inch. This screw is opposed to the micrometer screw, so that the 

 principle of repetition may be used in measures where two contiguous 

 lines of a scale appear in the same field of view. 



The fine adjustment is provided with an unusually stiff spring, to 

 VOL. XIII. (n.s. v.) 23 



