ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



517 



wood oil being recommended, as in arc-light it fluoresces without 

 further treatment. This little apparatus is set centrally on the con- 

 denser P. The dark-ground condenser is then so placed on the Micro- 

 scope stage that the middle of the prism A lies in the Microscope axis. 

 Capillary attraction draws the cedar oil up into the hollow space. A 

 narrow slit (fig. 81) is arranged under the condenser, the slit being 

 about 2 mm. broad between the two slips of cardboard, and lying 

 approximately centrally over the central diaphragm C. As the con- 

 densers are figures of rotation, it suffices to discover the ray-combina- 



Fig. 79. 



Fig. SO. 



tion in a plane-section containing the axis of rotation. Such a part is 

 stopped out by means of the slit S. AVhen the length of the oil- 

 chamber has been made to coincide with that of the slit, it is then 

 possible, by help of the prism A, to observe the course of the rays 

 directly with a loup, or with a low Microscope system. Two ray-courses 

 are observed near the prism : one from above, one from the side. 

 Fig. 80 gives a diagrammatic representation of these rays L, which 

 pass outside the central" stop C, and through the slip S enter the con- 

 denser approximately parallel. These come 

 to a focus at F in the cedar-oil space, and ' 3 



are then totally reflected at the cover-glass. 

 The prism A reveals these rays against 

 the dark background of the black glass G. 

 Arc-light must be used at a distance of 

 1-2 metres, and the horizontally incident 

 rays are not reflected up the Microscope 

 from a mirror (which usually gives two 

 images) but from a right-angled totally 

 reflecting prism ; in one series of observa- 

 tions the arc-light being focused by a lens, 

 the second series being without any such 



control. In the former case a larger illuminated view-field and a much 

 more intensive dark-ground illumination were obtained. The author 

 gives a plate of nine photomicographs showing the illumination under 

 different circumstances. The superior brightness in the cases when an 

 illuminating: lens was used is very noticeable. 



Fig. 81. 



