THE AMERICAN MONTHLY 



[April, 



upon the distance at which it is projected 

 from the eye. (a) The size at 25 cm.; (b) at 

 35 cm. 



Fig. 29 Figure showing the appearance of 

 the lines of the stage micrometer (the coarse 

 lines) and of the ocular micrometer when 

 using a high objective. 



A. One method of measuring the spaces by 



putting the ocular micrometer line opposite 

 the centre of the stage micrometer line. 



B. Method of measuring the space of the 

 stage micrometer by putting one line of the 

 ocular micrometer at the inside and one at 

 the outside of the lines of the stage micro- 



the cover-glass of the front lens of the homogeneous objective. 

 Remove the diaphragms from the illuminator or use a very large 

 opening. Focus the objective dowMi so that the immersion fluid 

 is in contact with both the front lens and the cover-glass, then 

 with the fine adjustment get the microbes in focus. Thev will 

 stand out as colored rods on a bright field. 



Shading the Object. — To get the clearest image of an object 

 no light should reach the e\'e except from the object. A hand- 

 kerchief or a dark cloth wound around the objective will serve the 

 purpose. Often the proper eftect mav be obtained by simply 

 shading the top of the stage with the hand or with a piece of 

 bristol board. Unless one has a very favorable light the shading 

 of the object is of the greatest advantage, especially with homo- 

 geneous immersion objectives. 



Cleaning Homogeneous Objectives. — After one is through 

 with a homogeneous objective, it should be carefully cleaned as 

 follows : Wipe ofi' the homogeneous liquid with a piece of the 

 Japanese paper, then if the liquid is cedar oil, wet one corner of 

 a fresh piece in benzine and wipe the front lens with it. Immedi- 

 ately afterward wipe with a dry part of the paper. The cover- 

 glass of the preparation can be cleaned in the same way. If the 

 homogeneous liquid is a glycerine mixture proceed as above, but 

 use water instead of benzine to remove the last traces of glvcerine. 



Acknowledgment. — To Dr. F. A. Rogers, Brewster, Mass., 

 for a remarkable slide, it being nothing less than a longitudinal 

 section of a human foetus of eight weeks. The specimen was 

 stained in bulk by borax-carmine, embedded in paraffine, sectioned 

 on a Bausch & Lomb Student microtome to 1-2000 inch, fixed on 

 the slide by an original and special method, stained as a contrast 

 with sulphindigotate of soda, and mounted in balsam, using 

 King's cement for ringing. To cut so large an object demands 

 considerable care and skill, and the entire preparation is labori- 

 ous and time consuming, but the result is woi'thy of attentive 

 study. Dr. Rogers has also kindly sent a section of the hand of 

 the same foetus, showing the developing muscle and bone. These 

 preparations are the finest of the kind that we have ever seen. 



Liquid Glue. — Soak glue in water, tlien melt it at moderate 

 heat, add vinegar until a thick fluid when cold. A small quan- 

 tity of acetic or of nitric acid will preserve the fluiditv at all ordi- 

 nary temperatures. 



