ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 621 



of brain sections. It is eqnipped with ii strong square sta^^'e 20 x 20 cm. 

 so that the largest sections can be conveniently exposed upon it. The 

 tube is of the same wide diameter as that sup])lied with the Stand A of 

 the same makers. The instrument is suitable for photographic purposes, 

 and, with weak powers, the whole section-surface can be photographed. 

 It is supplied with a line-adjustment, so that hi<rh power can be used if 

 desired. The coarse-adjustment is by rack-and-opinion. 



Swift's Large Measuring and Screw-testing Microscope. — This 

 instrument (ligs. 84, 85) has been designed to give absolute measure- 

 ments of small objects to a very fine degree with extreme accuracy. 

 It is particularly useful for measuring and checking such articles as 

 micrometer screws, divided scales, standard gauges, dies, etc., and is 

 constructed for great ease of manipulation in such work. It is designed 

 to give the length and pitch of a screw to O'OOl mm., the maximum, 

 minimum, and effective diameters and depth of thread toO'Ol mm., and 

 the angle of the thread to .5° without the necessity of moving the screw 

 after it has once been set up for examination. 



The object, according to its shape, is either held in one of the 

 chucks A of the rotatino- divided holder B, or fixed on the stao^e and 

 its length measured by moving it across the field of the webbed ocular 

 P by means of a micrometer screw with divided head C. The pitch of 

 this screw is 5 mm., and the head is divided into 100 parts : the frac- 

 tions of these divisions are read from a vernier to O'OOl mm. Entire 

 millimetres are shown by an index on the scale D. The plate of the 

 stage is held against the flint-hard point of the screw by two long 

 spiral springs set in the same place as the dovetailed fittings, one on 

 each side equally displaced. The point of the screw is turned on a 

 separate piece of steel to the thread ; it is hardened, ground and polished, 

 and let into the main piece before the thread is cut. This is done to 

 prevent distortion of the thread, w^hich would occur if the hardening 

 were done after cutting. The screw, which is of the most accurate 

 description, is cut between dead centres with a single point. 



The width of an object is measured by moving it across the field 

 by means of the milled head F ; the amount of the traverse is read to 

 • 01 mm. by the scale and vernier G. 



The angle between two lines, edges, sides, etc., as, for example, the 

 angles of a screw thread, is ascertained by rotating the webbed ocular. 

 One of the webs is brought coincident with a side, and the milled head 

 of the tang-ent screw H is turned until the web coincides with the other 

 side. The angle is given on the scale and vernier Iv to 5 . 



The milled head L actuates a tangent screw which inclines the 

 object under examination to the optic axis ; the degree of inclination 

 is read to 5° by the scale and vernier M. As the object lies in the 

 same plane as the axis of rotation it does not go out of focus on being 

 inclined. When the pitch of a screw is being measured, the screw 

 should be inclined the same number of degrees as the angle at which 

 the thread crosses it ; this angle can be approximated or else measured 

 accurately by means of the circle attached to the ocular. 



The object is focused by an ordinary rack-and-pinion coarse- 

 adjustment and a micrometer screw fine-adjustment ; the milled head X 



