ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



561 



drawing plane. For diminution B is made the object plane and A the 

 drawing plane. For drawing in natural size the lenses are dispensed 

 with, and the object is placed at A 250 mm. from the eye ; the drawing 

 plane must be at the same distance. Between the prism and the mirror 

 is 105 mm., so that the drawing plane must lie 145 mm. from the mirror. 

 It is also possible to arrange the apparatus for drawing objects in a 

 vertical position. 



Some Improvements in T. Tammes' Electric Microscopical Lamp.* 

 T. Tammes has introduced several improvements into his lamp. The 

 top of the cast-iron frame in which the incandescent lamp was originally 

 contained is now replaced by a brass plate secured by milled heads to 

 the sides. By relaxing these screws insertion of a new lamp is facilitated. 

 In the centre of the top is a knob through which passes the small wire 

 cable. This cable enters the knob horizontally, as it was 

 found that a vertical entrance had a tendency to entangle Ernst Leitz 

 the wire with the Microscope objective. The knob is also Wotzlap. 

 useful as a handle for carrying the apparatus. The back 

 of the box is now made of blackened copper, which has 

 the effect of destroving reflexions. 



Leitz Step Micrometer with Simplified Micron 

 Graduation. f—C. Metz describes this accessory (fig. lOU) 

 which appears to possess several advantages. The 

 micrometer is photographed deep black and the strokes 

 stand out so clearly as to be easily seen and yet are not so 

 thick as to interfere with the measurement. The divi- 

 sions are grouped in tens, each group being plainly 

 distinguished from its neighbour by reason of the step- 

 like arrangements of its units. The divisions are twice 

 numbered — black on white and white on black — so as to 

 be readily distinguishable in any light. The micrometer 

 comprises fourteen groups of which only the ten middle 

 ones serve for the finer countings, because in the centre 

 the field is sharper, and greater freedom from error is 

 required than at the circumference. The graduations 

 are closer than usual, the intervals being 0"06 mm. This 

 value is selected because with tube-lengths not greatly 

 varying from normal the micrometer values of all object- 

 ives (achromats, fluorites, and apochromats) can be ex- 

 pressed in easy numbers which facilitate calculation. It 

 is well known that the micrometer value of an objective 

 is the number of divisions of the image covered by one 

 division of the ocular micrometer. Thus, for example, 

 if in the case of an ordinary ocular micrometer divided 

 into tenths of a millimetre, this value were 0*00349 mm. = 3-4'.» yu, 

 (as it would be with Leitz objective No. 6 and tube-length 160 mm.). 



Fig. 100. 



* Zeitschr. wiss. Mikrosk., xxix. (l'Ji2) pp. 82-4 (1 fig.). 

 t Zeitschr. wiss. Mikrosk., xxix. (1912) pp. 72-8 (1 fig.). 



