Febeuakt 17, 1911] 



SCIENCE 



249 



leyan University, received his Ph.D. in 

 physiology at Chicago in 1902 and his M.D. 

 from Eush Medical College in 1905. Before 

 going to St. Louis he was instructor in physi- 

 ology in the University of Chicago, and at the 

 Marine Biological Laboratory at Woods Hole. 



Mrs. Margrete Bose has been appointed 

 professor of chemistry in the University of 

 La Plata. Her husband. Dr. E. Bose, is pro- 

 fessor of physics in the university. 



Mr. James Lees, of the University of Bris- 

 tol, has been appointed lecturer in engineer- 

 ing in the South African College, Cape Town. 



Dr. Franz Hofmann, professor of physiol- 

 ogy at Innsbruck, has been called to the Ger- 

 man University of Prague to succeed Pro- 

 fessor Gad. 



DlSOnSSIOW AND CORRESPONDENCE 

 laboratory tables 



To THE Editor of Science : Several months 

 ago I read with interest in Science Professor 

 Augustus H. Gill's suggestions for chemical 

 laboratory furniture and fittings. Among 

 other things he discusses various kinds of 

 materials and surfaces for table tops. It oc- 

 curs to me that it may be of sufiicient inter- 

 est to warrant calling attention to still 

 another kind of surface for laboratory tables. 



In our testing laboratory at The York 

 Manufacturing Company we have tables with 

 tops of ordinary wood. On this there are 

 placed sheets or slabs of heavy asbestos board, 

 one fourth inch thick. These are fastened in 

 place by a few small brads driven around the 

 edge. All around the outer edge of the table 

 there is a narrow strip of wood of the same 

 thickness as the asbestos board, making a 

 permanent border. This, as a matter of 

 course, is nailed in place. It prevents the 

 edges of the asbestos from becoming frayed 

 out. The advantages of this asbestos surface 

 are almost self evident. Flasks and beakers 

 containing hot water or solutions can be stood 

 upon it without fear of their cracking. 

 There is also little risk of breaking glassware 

 by setting it down a little too hard, as is 

 often the case on slate or stone or even wood, 



where particles of grit may happen to be. 

 And of course the resistance of the asbestos 

 to fire and heat is too well knovm to need any 

 comment. There is the further advantage that 

 when the asbestos slab becomes old and worn 

 it is easily replaced without in any way dis- 

 turbing the table, thus making the latter 

 practically new. 



We have found this plan highly satisfac- 

 tory and pass on the suggestion for any who 

 may desire to try it. It is quite possible that 

 it is an old device after all. 



C. H. Ehrenfeld 



York, Pa. 



^ a formula foe optical instruments 



In many surveying and optical instruments 

 a ray of light is reflected by a pair of plane 

 mirrors. And if <f> be the angle between said 

 mirrors; and the entering light ray lies in 

 the plane commonly perpendicular to them; 

 then, of course, the doubly reflected ray must 

 cross its original path at the angle 2(^. And, 

 although the ray sway from side to side; so 

 long as it preserves its parallel position to this 

 commonly perpendicular plane; so long also 

 is the crossing angle still 2<^. 



But now, should the entering ray be de- 

 flected at a variable angle 6 to this commonly 

 perpendicular plane, then the question arises 

 as to the resultant effect upon the crossing 

 angle, a problem that constantly arises in 

 practise, and yet one, I believe, that the text- 

 books leave unanswered. 



The single solution is as follows : letting <j> 

 be the angle between the two mirrors, and 6 

 be the independent variable angle that the 

 entering light ray makes with the plane com- 

 monly perpendicular to the said mirrors, 

 while 8 is the crossing angle desired. Then, 



(cos 8) (sin<^) =sin J5, 

 a very simple formula, that can be easily 

 demonstrated by elementary trigonometry. 



In the special cases where the entering ray 

 is normally inclined to the commonly perpen- 

 dicular plane, and it be asked what errors may 

 be produced by changes in the direction of 

 that ray? we should simply determine, first, 

 the angle <^ between the two mirrors, and 



