ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



119 



the draw-tubes of all sizes of stands are beginning to be fitted with 

 the " Society " screw, to carry objectives at the lower end. This is a 

 practical innovation, which might be generally adopted, we think, 

 with advantage. The object is to enable low powers — such as 3 inch, 

 4 inch, or even 5 inch — -to be readily used, the draw-tube providing 

 convenient and rapid means of focussing roughly, without the body- 

 tube itself having to be moved higher than the normal position, where 

 the Microscope is well balanced. 



Accurate focussing can be obtained with large stands, by the usual 

 coarse rack and pinion motion. The nose-piece of the main optical 

 body (into which the higher powers are screwed) is not found to pre- 

 sent any obstacle to the use of the draw-tube. 



There is the further advantage in a screw on the draw-tube, that 

 an objective can be applied to produce with the ordinary eye-piece an 

 erecting arrangement, so that a special " erector " may be dispensed 

 with. This, as is well known, enables wide variations in magnifying 

 power with the same objective to be obtained, and is not only service- 

 able for dissections, but for the examination of gems, large specimens, 

 and low-power work generally. Amplifiers may also be attached to 

 the lower end of the draw-tube. 



Dr. Royston-Pigott's General and Transfer Finder. — The fol- 

 lowing description is supplied by Dr. Eoyston-Pigott : — The finder 

 (Fig. 11) consists primarily of sis large triangles lettered A, B, G, 

 D, E, and F at the margin. Each triangle is divided into thirty-six 

 small triangles lettered or figured. Any triangle is identified by two 

 letters, such as Bz, Ea;, or Fr. 

 A platinum disk perforated with 

 a small hole is made to slide or 

 fold over into the position for 

 zero. This is used only for 

 finding objects generally. The 

 triangulated finder is transpa- 

 rent, and is placed so as to occupy 

 the stop in an A eye-piece. 



Transfer Finder. — Plan. — 

 The finder should just fill the 

 stop of an A eye-piece, which 

 must have a stud to correspond 

 with the notch on the draw- 

 tube, so as to keep the finder 

 vertical in one position, and 

 the notch should go through the 

 eye-tube and draw-tube together. 



Use. — To transfer say an object from an -I- to a ^^ infallibly. 



1. Use a standard inch always to mark the proposed 1 or -^-^ 

 or ^V- Place a sharp object in the centre by the i inch and finder. 

 Then substitute an inch objective, and mark the position, say Bz, on 

 the finder. Upon placing any other object on B2 with the inch, the 

 given \ will show it in the middle of the glass. 



2. Suppose it is required to find the triangle for a ■^^. Take 



Fig. 11. 



