1898] MICROSCOPICAL JOURNAL. 11 



PRACTICAL SUGGESTIONS. 



By L. a. WILLSON, 

 cleveland, ohio. 



Micrometry. — "While all the principles of microme- 

 try are simple, it is very diflBcult to get the exact size of 

 microscopic objects. This is due to the lack of perfec- 

 tion and uniformity of micrometers, and the diflBculty in 

 determining the exact limits of the object to be measured. 

 Hence, microscopic measurements are only approximate- 

 ly correct, the error lessening with the increasing perfec- 

 tion of the apparatus and the skill of the observer. It is 

 said that 0.2 of a micron is the limit of precision in mi- 

 croscopic measures, beyond which it is impossible to go 

 with certainty,". — .Grage. 



GrROUND-GLASS SLIDE. — In using the dissecting micro- 

 scope with a mirror there is generally too much glare. 

 This can be obviated if we intercept the light by usino- a 

 ground-glass slide on the stage. The light will then be 

 diffused and work may be accomplished with comfort. 



PODOSPHAERA BIUNCINATA, — This is One of the fungus 

 species of the family Erysiphae. The beautiful and in- 

 teresting plants of this family have now ripened and the 

 autumn leaves are full of them. They are readily gath- 

 ered and easily manipulated. All that is necessary is to 

 scrape off a few of the little dots, place them on a slide 

 with water, cover, then see that the space under the cover 

 is filled with water. Examine with a power of an inch, and 

 if desirable, afterwards use a power equal to a quarter. 

 Remove the dots carefully from the leaf and be careful 

 not to roll the specimen up with the spider-web-like my- 

 celium. The round dots are the perithecia which con- 

 tain the asci with spores. To see the latter, press on the 

 cover glass and gently split the asci. No reagent of any 

 kind should be used. Podosphara biuncinata is a strik- 

 ing species. The perithecas contain but a single ascus 



