SCIENCE 



Nbw Sebies 



Vol. LIII, No. 1360 



Friday, January 21, :-j ^21 



Single Copies, 15 Cts. 

 Annual Subscription, 96.00 



Concentration 



It is difficult, by the spoken word, to get the undivided attention 

 of all students. It is easy to get complete concentration on a 

 lecture when words are supplemented by illustrations projected 

 with a 



Bausch & Lomb Balopticon 



The Perfect Stereopticon 



Balopticons project slides, opaque objects or both with remarkable 

 ease of operation. Photographs, post cards, specimens, illustra- 

 tions, maps, etc. , are clearly shown by the dependable, even and 

 safe light of the new gas-filled Mazda lamp. 



Bausch ^ Ipmb Optical ©. 



ROCHESTER, N. Y. 



Ne'w York 



Washington 



San Francisco 



London 



552 St. Paul Street 



Chicago 



Leading American Makers of Photographic Lenses, Microscopes, Projection-Apparatus 

 (Balopticons), Ophthalmic-Lenses and Instruments, Photomicrographic Apparatus, Range 

 Finders and Gun Sights for Army and Navy, Searchlight Reflectors, Stereo-Prism 

 Binoculars, Magnifiers, and other high-grade Optical Products 



