ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



959 



Fig. 153 shows the arraugemeut as first aj^pliccl with three racks, the 

 teeth of each part being set out of line to the extent of one-third 

 their pitch and the spiral pinions being fitted to correspond with the 

 racks. The effect is similar to what would be obtained by pitching 

 the teeth of a single rack three times as finely, but at the same time 

 retaining the strength due to the coarser pitch. 



Mr. Mayall subsequently suggested that as the fitting of three 



Fig. 153. 



Fig. 154. 



Fig. 155. 



pinions on the same axis presented some difiiculties of workmanship, 

 these might be considerably reduced by using two racks instead of 

 three, as shown in fig. 154, still retaining a considerable advantage 

 over the ordinary single diagonal rack. 



Fasoldt's Nose-piece.— Mr. C. Fasoldt's form of nose-jjiece (fig. 155) 

 is somewhat similar in principle to that suggested by Mr. Curties.* 

 The Society screw to receive the objective consists of three segments, 

 one of which is on a movable piece which is 

 acted on by a spring lever, a jam-nut enabling 

 the lever always to be set at the most con- 

 venient point for working it, say in front of 

 the body-tube. On pressing the lever the 

 objective can be introduced, and if inserted so 

 that the threads correspond, will not require 

 any turning, but otherwise a fraction of a turn 

 may be necessary. " The position of each 

 objective when screwed up is readily found, 

 and can then be marked so that it may always 

 be inserted near this position." f The latter requirement would seem 

 to introduce an element of difficulty, which it is the essential object 

 of such contrivances to eliminate. 



Spencer's Dust-protector for Objectives.+ — H, K. Spencer & Co. 

 have patented a device to protect the interior and backs of objectives 

 from dust. It^ consists of a thin piece of plate glass polished, and 

 mounted in a ring screwed into the back of the objective. It is 



♦ Rec this Jouriml, iii. (1883) p. 572. 



t Of. Micr. Build in, i. (188t) pp. 42-3. 



i Amcr. Mon. Micr. Joiirn., v. (1884) p. 200. 



