516 



SUMMAEY OF CURRENT RESEARCHES RELATING TO 



being parallel to the rotation axis of the telescope. A drop of oil or 

 water secures optical contact between the prism and glass plate. 



Marpmann — Ueber Prismenspektroskope und Mikrospektroskope mit Fernrohr. 

 (On Prism-spectroscopes and Micro-spectroscopes with Telescope,) 



Zeit.f. angew. Mihr., V. (1900) pp. 309-13 (2 figs.). 



(6) Miscellaneous. 



Becks' Cover-glass Gauge. — Since the thickness of the cover-glass 

 slightly alters the correction of a high-power object-glass, the purpose 

 of this gauge (fig. 134, one-third full size) is for observing the thickness 

 of thin glass covers under which objects are to be mounted. 



All the object-glasses of these makers which are not provided with 



Fig. 134. 



^^^inmfniwiiiimi"!"'""' wii HiiiSTiMSSiiiiiifiSniin' 1 ''"" 



correction collars are corrected for cover-glass of • 006 in. ; this is 

 the mean thickness of their No. 1 cover-glass, and it is advisable that 

 no glass thicker than this should be used for high-power observation. 



To ascertain the thickness of a cover-glass or lamina of any kind, 

 first raise the long lever, and then slide in the cover-glass on the steel 

 plate or bracket, and gently lower the lever on to it ; the hardened 

 steel point close to the fulcrum of the lever must rest on some portion 

 of the cover-glass. 



The thickness can then be read off on the scale in ■ 001 in. ; thus 

 the scale illustrated above reads 0*006 iD. 



Fig. 135. 



Mayer's Simple Object-pusher.* — Dr. Paul Mayer, of Naples, makes 

 this convenient accessory (fig. 135) out of a thin metal plate, whose edges 

 are cut accurately enough to move freely between the bases of the ordi- 

 nary object-clamps (fig. 136). One spring is removed, and the other 

 serves to press the plate on the stage with suitable pressure. The upper 



* Zeit. f. wiss. Mikr., xvii. (1900) pp. 7-9 (2 figs.). 



