518 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Work with the Paraboloid Condenser.* — W. Scheffer communicates, 

 with much technical detail, an account of certain work with the Zeiss 

 paraboloid condenser. The plate appended to his paper consists of six 

 instantaneous photographs of various objects (spermatozoids, blood- 

 corpuscles, etc.), some of which were photographed while in active 

 motion. 



Hensoldt and Sons' New Angle Prisms for 90°, 180°, 45°, and 

 Roof-prism.! — The "Wetzlar firm strongly recommend these prisms as 

 being much superior to the glass mirrors frequently used in optical 

 instruments. Their special advantages are : (1) Very large field of 

 view ; (2) absolute accuracy ; (3) invariability ; (4) quick orientation 

 with regard to the object. The manner of use is to obtain coincidence 

 of object and the image, as is often done by mirrors in instruments of 

 older type. 



Fig. 82 shows the " pentagon " prism, which is adapted for deviating 



Fig. 82. 



Fig. 83. 



a ray through 00°. The path of the ray proceeding from o and falling 

 perpendicularly on the surface b a is easily traced on the diagram. It 

 will be noted that the prism is made of one piece. 



Fig. 83 shows a combination of two " pentagon " prisms reversed 

 and rotated through 90°. This arrangement gives a deviation of 180°. 



Fig. 84 shows the " pentagon " prism bisected through i and inverted. 

 If now the observer looks over the prism in the direction ac at an 

 object p he will find p coincide with the image of o seen by refraction 

 and reflection along the broken line o grst. In this way an angle of 

 45° was obtained. 



* Zeitschr. wiss. Mikrosk., sxv. (1908) pp. 446-50 (1 pi.), 

 t Catalogue, M. Heusoldt und Sonne, Wetzlar. 



