1898J MICROSCOPICAL JOURNAL. 11 



PRACTICAL SUGGESTIONS. 



By L. a. WILLSON, 

 cleveland, ohio. 



Micrometry. — "While all the principles of microme- 

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

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

 tion and uniformity of micrometers, and the difficulty 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." — Gage. 



Ground-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 Erysiphse. 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. Podosphaera biuncinata is a strik- 

 ing species. The perithecas contain bat a single ascua 



