and Laboratory Methods. 2663 



asphalt solution has in slight degree the qualities desired in an imbedding medium 

 for brittle objects. A more important quality is that it is more transparent than 

 parafifin and so facilitates orientation of the object for cutting. Finally, the 

 paraffin-asphalt-rubber solution combines the qualities of toughness and trans- 

 parency, so that it will probably be the most valuable of the media described 

 here. 



The cost of either of these media need not exceed that of ordinary paraffin 

 more than one cent per pound. I have not been able to test asphalt from dif- 

 ferent sources, but do not anticipate that there would be any practical difference. 

 It remains to be determined by the experiments of others whether these media 

 will injure delicate cytoplasmic structures. The small amount of rubber and 

 asphalt used would not lead me to expect such difficulty. 

 West Virginia University. J. B. JOHNSTON. 



An Artificial Light for the Microscope. 



During a considerable part of the year daylight is often insufficient for suc- 

 cessful work with the microscope. Numerous contrivances for artificial illumin- 

 ation have been devised, some of them fairly good, but most of them thoroughly 

 unsatisfactory. During the past two years experiments in artificial illumination 

 have been conducted in the botanical laboratory of the University of Chicago, 

 and as a result we now have a cheap and practical light. The idea is not at all 

 new, since a somewhat similar device was used by Hooke more than two hun- 

 dred years ago. Practically the same apparatus is now in use at Prof. Stras- 

 burger's laboratory in Bonn. 



The apparatus consists, essentially, of a hollow sphere filled with liquid. A 

 fairly good and practical light can be gotten with an ordinary lamp by 

 allowing the light to pass through a wash bottle filled with a weak solution of 

 ammonia copper sulphate. A piece of dark paper with a circular hole in it serves 

 as a diaphragm and at the same time protects the eyes from the direct light of 

 the lamp. Such an arrangement is shown in Fig. 1. Wash bottles, however, 

 are not perfectly spherical and the mounting is not convenient. To secure a 

 perfectly spherical globe, it was necessary to have a mold made. The globes, 

 as we now use them, are of the finest flint glass, have a diameter of six inches, 

 and are mounted in a convenient black frame. Fig. 2. The globe acts not only 

 as a condenser, but also as a ray filter. For general laboratory work and for 

 nearly all research work, a weak solution of ammonia copper sulphate has proved 

 most satisfactory. The solution (to fill one six-inch globe) may be made by 

 adding 50 c. c. of ammonia to 25 c. c. of a 10 per cent, solution of copper sul- 

 phate and then adding enough distilled water to fill the globe. If a white 

 precipitate appears and makes the solution look milky, add more ammonia. The 

 strength of the solution depends so much upon the power of the light that no 

 fixed formula can be given. We simply dissolve in water a small crystal of 

 copper sulphate — about as large as a grain of corn — then add about 50 c. c. of 

 ammonia, and then add distilled water until alight, clear blue solution is secured. 



