The Microscope. 159 



accomplished without much difficulty. There is also an adap- 

 tation for ruling the longer and comparatively coarser lines for 

 diffraction plates for spectroscopes. 



WHY DO DRY MOUNTS FAIL? 



BY M. A. BOOTH. 



In looking over my collection of slides, procured both by 

 purchase and exchange, and representing the work of European 

 and American preparers, with a view to noting their keeping 

 qualities, I have been so surprised at the number of failures as 

 to query whether permanence in microscopical work is possible. 

 Why is it that so large a proportion of dry mounts — the first for 

 the beginner to essay and the simplest, the mere protecting the 

 object from dust and moisture — fail? Obviously that motto 

 which should emphatically be the microscopist's motto, " Fes- 

 tina lente" is not heeded by all workers. The advances in the 

 merely mechanical portions of mounting have evidently not 

 kept pace with those in its purely scientific departments, or 

 else microscopists sometimes forget to take counsel of their 

 good common sense in the use of cement. In this collection are 

 slides which have cost hours of skillful manipulation and yet 

 are utterly ruined because of inattention to the details of the 

 proper use of cement. How do we sometimes apply balsam to 

 a mount? By running it over the cover and trusting to capill- 

 ary attraction to fill the field. But why should this law of 

 capillary action be operative in the case of the balsam and sus- 

 pended in that of the cement? 



From careful observation and a not limited experience — I 

 speak of dry mounts of diatoms and the like — I am convinced 

 that success or failure depends not so much upon the kind of 

 cement used as upon the care with which it is used. 



In my own work, however, I have fixed upon white zinc as 

 the most reliable cement, and I have sent out hundreds of slides 

 made with this cement accompanied with the request that all 

 failures be returned, so that I might replace them with perfect 

 slides ; but not a slide has ever been returned. It has been my 

 experience that white zinc properly prepared and properly used 

 never fails. The secret of success with good white zinc, is that 



