ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



501 



combined with the best optical appliances. As regards the second 

 requirement, four kinds of projection have been provided — viz. dia- 

 scopic, epidiascopic, megascopic, and microscopic. In the diascopic 

 installation, diapositives up to 13 by 18 cm. can be used, and at a 

 distance of 5 metres from the objective a magnification of 14 diameters 

 is obtained. The epidiascopic and megascopic projections produce a flat 

 surface of uniform expansion. The body of the apparatus is set on a 

 strong cast-iron frame running upon rollers, and stiffened by a wooden 

 inclosed utensil box. The projection apparatus is supplied with an 

 automatic self-regulating arc lamp of special construction. The lamps 

 are designed for a uniform current strength of 30 amperes. The upper 



1 



Fig. 129 



and positive carbon, whose crater acts as the light source, is fixed in the 

 optic axis, thus giving the great advantage of constant centricity as the 

 carbon burns away. Moreover, as this crater is applied directly to the 

 illuminating apparatus, a uniform current furnishes a higher intensity 

 than is obtained with lamps of older make. As the negative carbon is 

 vertical, the light source can be brought very close to the condenser, 

 thus yielding another advantage, because the condenser can thus be 

 made of higher aperture — a distinct gain to the brightness of the image. 

 The special features of this lamp, therefore, make it very easy and con- 

 venient to manage ; it moves on runners, and can be fixed by clamp- 

 screws ; there is a lever for operating it in the direction of the optic 

 axis. Fig. 129 gives a good general view of the apparatus as a whole. 



