304 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



divided drum, each graduation on the drum signifying one-hundredth 

 of a millimetre. The movement is limited by two hard stops. The 

 beam of rays diverging from the entrance-slit is parallelized by the 

 achromatic collimator objective inside the box and is then transmitted 

 through the prism : these rays then pass through a telescope objective 

 and throw a spectrum (visible length 12 mm.) in the plane of the exit- 

 slit. By rotating the prism the whole spectrum can be made to traverse 



I" 



Fig. 32. 



the slit so that any desired part can be brought into use. The actual 

 position of the prism can be read off on a disk. It is possible to 

 standardize the graduation according to the wave-length. The breadth 

 of the exit-slit is usually twice as much as that of the entry-slit. It is 

 very important to arrange the height of the apparatus so that 

 the emergent rays exactly coincide with the axis of the Microscope 

 mirror. The apparatus is mounted on a wooden base so that the 

 Microscope can be moved in two mutually perpendicular directions. 



(5) Microscopical Optics and Manipulation. 



Demonstration Experiment on the Abbe Theory of Microscopical 

 Perception.* — H. Ambronn describes a series of experiments he 

 iias undertaken for the purpose of illustrating Abbe's doctrine that 



* Zeitscbr. wiss. Mikrosk., xxx. (191 4) pp. 289-99. 



