ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



351 



ing the instrument into circuit with an electric current obtained from a 

 dry cell or other convenient source. The hollow mirror can be pushed 

 up and down the pillar and clamped by a screw St. When the lamp- 

 filament is brought into the focus of the mirror an intensely bright stream 

 of parallel rays is directed outwards. The lamp can be set immediately 

 under the condenser ; or the Microscope mirror, if irremovable, can be 

 set at a proper angle for receiving the light horizontally and reflecting 

 it vertically upwards. A coloured disc can be set in the condenser if 

 desired. Simple means are provided for regulating to a nicety the light- 

 intensity. A bibliography on electric lamps is appended to the original 

 article.* 





a 



e 



Fig. 61. 



Engelmann's Micro spectral Objective with Detachable Thorp's 

 Grating and Detachable Polariser.f — The invention of the Thorp 

 transparent grating has put a new agency at the disposal of spectroscope 

 makers. The rulings are about 14,5G0 to, an inch, and the intervals are 

 1 • 7 fx. In consequence of this the perpendicularly incident principal 

 rays in the first diffraction spectrum are deviated about 20° for central 

 yellow. The application of such a transparent plane grating in the 

 microspectral objective would have required a corresponding inclination 



* As the rays from the electric lamp are divergent, and those from the parabolic 

 mirror parallel, they cannot both be brought to a focus on the object, at the same 

 time, by the substage condenser. — [Ed.] 



t S.B. k. preuss. Akad. Wiss. zu Berlin, xxxi. (1902) pp. 711-9 (7 figs.). 



2 a 2 



