94 THE AMERICAN MONTHLY [Mar., 



mislaid, are lost. Magazines are filed and can be found for ref- 

 erence. But one announcement is not sufficient. There 

 should be something in every issue from April to August about 

 the approaching meeting. This year we intend to have some- 

 thing about the Society in every issue from February to August 

 whether the Secretary furnishes it or not. We can if need be 

 make bricks without straw. 



MICROSCOPICAL APPARATUS. 



Ross's Petrological microscope. — This micioscope is de- 

 signed to provide a thoroughly reliable instrument for students 

 in petrology. In size and form it resembles the "Eclipse" 

 pattern (with reversible foot to ensure stability in any position). 

 The stage is circular, revolving, and the periphery divided into 

 360°. The analyser, which can be drawn out when not needed, 

 is fitted into the lower end of the body tube, where also a slot 

 is cut at the angle of 45° for the insertion of a quartz wedge, etc. 

 The polarizer is pivoted to swing out of the field when so re- 

 quired, and it has a circle divided into 8, clicked at 0° and 180° 

 to indicate when the Nicols are crossed. The eye-piece is fur- 

 nished with crossed webs aad readily drops into a slot. The 

 milled-headof the micrometer screw is divided to measure 1-500 

 of an inch motion of the fine adjustment. Plane and concave 

 mirrors are provided. The price of this capital instrument, 

 with one eye-piece, I2 and ? object glasses, double nose-piece, 

 (adjusting object glasses to same focal plane), polarizing and 

 analysing prisms, and Klien's quartz plate, mahogany case, is 

 $50. A number of accessories fitted to the petrological micros- 

 cope are listed by the makers, Messrs. Ross, of New Bond 

 Street, at very moderate prices. — I'he Optician, London. 



MEDICAL MICROSCOPY. 



The Duration of the Contagiousness of Diphtheria. — 



The Journal des Praticiens for January 19th contains an article 

 in which the writer remarks that this question is the subject of 

 much controversy. A young physician, M. Tezenas, recently 

 collected sixty observations of patients who were attacked with 

 characteristic diphteria, and he divided these cases into three 



