124 THE MICROSCOPE. 



hundreds of doctors into our ranks as active workers. The re- 

 sults would benefit not only the society but also the practition- 

 ers. 



QUESTIONS ANSWERED. 



Note. — Dr. S. G. Shanks, of Albany, N. Y., kindly consents to receive all sorts of ques- 

 tions relating to microscopy , -whether asked by professionals or amateurs. Persons of ah 

 f ades of experience , from the beginner upward, are wlcome to the benefits -f this depart- 

 ment. The questions are number id for future i eference. 



164. How is Hoyer's picro-carmine made, and hoiu used f — L. W. 



Take powdered carmine 2 grains, water 1 ounce, aquae-am- 

 monite enough to dissolve the carmine while heated in a cap- 

 sule, boil gently until the ammonia has evaporated, cool and 

 filter. Make a saturated solution of picric acid iu water, take 

 4 ounces of it and mix with the above carmine solution, filter^ 

 add a crystal of thymol, or about 1 per cent of chloral hydrate' 

 to prevent change. 



Stain sections 3 to 10 minutes. Overstaining may be ex- 

 tracted with 70 per cent alcohol, then use absolute alcohol, oil 

 of cloves, balsam. 



165. Can you giv? the addresses of dealers in the aniline dyes 

 used for staining f — L. W. 



Any of the dealers in microscopical supplies can furnish the 

 aniline stains. Consult our advertising column and send for 

 catalogues. 



166. Explain simply and clearly, how to obtain the numerical ap- 

 erture of an objective. — B. E. R. 



The proper method is to use the Abbe apertometer, an in- 

 strument made for measuring N. A., and indispensable for the 

 measurement of high angled objectives. The following method 

 can be used to measure the angle of aperture of dry objectives. 

 Place a microscope on a table or board, screw the objective in 

 place and turn the tube down to the horizontal. Take a lath or 

 similar strip of wood, 2 or 3 feet in length, drive a pin or wire 

 nail through one end and into the table directly beneath the 

 front lens of the objective. Arrange a small candle on the free 

 end of the lath, so the flame will be at the same height orlevel 

 as the objective, and can swing around the objective as a center. 



