182 THE AMERICAN MOIS'THLY [Oct. 



live Committee, Professor S. H. Gag-e, of Ithaca ; Dr. A, 

 Clifford Mercer, of Syracuse, and Dr. V. A. Moore, of 

 Ithaca; 



Lime Light, ^ — The surface of the lime is not so evenly 

 incandesced as that of carbon. It g^ives a small homoge- 

 neous point of lig-ht and quite intense. But by reducing- 

 the pressure of the gas to about one inch and using- a very 

 hard lime and a jet with a medium-sized bore a fairly 

 steady light is obtained. Still the arc lig-ht is better. 



Electric Arc Lamp. — There has been g-reat difficulty in 

 using this lig"ht for photomicrog-raphy because the position 

 of the arc was not constant and the source of lig-ht not 

 uniform, Messrs. Barnard and Carter of the Quekett 

 Club devised a form in which the distance apart of the 

 carbon points is regulated by hand, their position thus con- 

 trolled easil}^ By reference to cross-wires on a glass 

 screen the source of light can always be kept in the same 

 place. The oblique position in which the carbons are set 

 enables the small point of intense light from the incan- 

 descent crater of the positive carbon is used as a source 

 of unvarying and steady illumination of small area and of 

 very great intensity. 



Dry Mounts. — The Postal Microscopical Club people 

 have been discussing dry mounts and appear to disap- 

 prove of them as liable to deterioration. The great differ- 

 ence of refractive index between the objects and the air 

 which makes them nearly opaque from total internal re- 

 flection of the light results in foggy images and often 

 totally obscures the structure. A medium of higher 

 refractive index sliould be used for hyaline forms like 

 diatoms, even higher than balsam for very thin forms. 

 During mounting the organic matter is burned out by 

 heating the cover-glass. During this pi'ocess, some of the 

 forms may be melted into the cover-glass. They may be 

 observed profitably if the glass be made with the same 

 refractive index as that of the front lense of the objective. 

 A common accident to the mount is the running in of the 

 cements but this accident sometimes changes the dry 



