ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



647 



w^ 



Fig. 103. 



vertical axes at W, X, Z, Y, above a drawing board ; at / is a rod, held 

 by drawing pins, which serves as the fixed point about which the whole 

 instrument turns in drawing ; the paper is placed under the pencil at 6 ; 

 the object is at d under a lens which is carried by a 

 diopter in Z Y, and which has a cross engraved upon 

 its upper surface. The pencil at 6 is moved by the 

 hand in such a way as always to keep the centre of 

 the cross upon the outline of the object as it appears 

 to the eye above d. The scale of the drawing may 

 be varied by sliding the rod / and the lens d along 

 their bars ; the points fdh are to be always in the 

 same straight line. Between V and Z are slots cor- 

 responding to an amplification of 2, 5/2, 3, 4, 6, 8, 

 and 10 respectively, and the lens is adjusted by 

 means of a scale along Z Y having its zero point at V 

 Z. The board being set horizontal by a level, and 



the upper opening of the tube being adjustable, the line joining the two 

 openings may be made vertical by a plummet, so that the line of vision 

 is always perpendicular to the plane of the drawing. The lens is made 

 horizontal by means of a pendulum movement about the screw which 

 fixes the lens-holder to the tube. The lens may also be adjusted by 

 means of a horizontal mirror placed below it, the engraved cross being 

 made to coincide with its image seen in the mirror. When higher 

 powers are used the object is to be raised by a support to the correct focal 

 distance. When a large object is being drawn, the long axis at Y may 

 be replaced by a short one ; and in this case the bar X Y may be pro- 

 longed beyond Y, and fitted with a long axis at its end. 



The instrument is designed "rather with a view to practical con- 

 venience than to realize with mathematical accuracy the exact repro- 

 duction of an object." 



Slide for observing Soap-bubble Films.* — A simple means for 

 showing soap-films by the Microscope, may, Mr. F. T. Chapman points 



Fig. 104. 



out, consist of a thin strip of wood (3 in. by 1 in.), or other material, 

 with a metal plate secured to it. The plate should have one end 



* Eead before the Washington Microscopical Society. Cf. Amer. Men. Micr 

 Journ., ix. (1888) pp. 81-2 (1 fig.). 



2 Y 2 



