1882.J 



MICROSCOPICAL JOUENAL. 



77 



Chytridium is a curious plant which 

 attacks not only desmids, but other 

 algse, and also the saproligniese, but 

 we have not been able to find a single 

 species described as infesting Clos- 

 terium, in any book in our posses- 

 sion. 



o '■ 



Collecting. — During March, April 

 and May, many of the algse belong- 

 ing to the family zygnemacese will be 

 found in fruit when they are most valu- 

 able in collections. Vaucheria fruits 

 early, and doubtless can now be 

 found in good condition to study. Last 

 month Mr. Wolle found new growths 

 of Chcetophora, Ulot/irix, Batrachos- 

 permum, Oscillaria, and other algse. 

 At a meeting of the New York Mi- 

 croscopical Society, some newly 

 hatched snails were shown, and the 

 eggs should now be abundant on wa- 

 ter plants, in almost every pond. 

 Frog-spawn also must soon be found; 

 and it is always interesting to watch 

 the development of the tadpoles and 

 their slow change into frogs, all of 

 which can be followed in a jar of wa- 

 ter. Euglenas, vorticellas, rotifers, 

 water-bears, floscules, hydras, and a 

 host of other specimens can be found 

 in abundance this month. Mr. Ba- 

 len's list gives the names of the spe- 

 cies he has found quite lately. 



For the novice in collecting we 

 make a few suggestions. Look along 

 the borders of ponds where weeds 

 and grass are growing submerged. 

 Pull out some of the stems that are 

 covered with a light, flocculent de- 

 posit and examine them. Dip down 

 into the bottom of the pond, and pull 

 up whatever sticks and old tree 

 branches, or plants, that may be 

 caught. Look them over with a hand- 

 lens for gelatinous masses and grow- 

 ing alg^, or whatever of interest they 

 may have. For water-fleas, small 

 entomostraca and water-insects, run 

 a small hand net, made of common 

 cloth, along near the bottom of the 

 pond, and many can thus be caught. 

 Volvox, which seems to appear at all 

 seasons, can also be collected in such 



a net, which need not be more than 

 three inches in diameter. 



Ruled Lines as Tests. — Last 

 month we made some allusions to 

 test-plates of ruled bands, and pro- 

 mised a few words more in this num- 

 ber. With reference to the plate 

 ruled by Prof. Rogers, we have since 

 obtained from him the estimated 

 width of the individual lines. Ac- 

 cording to this estimate, in his band 

 which was ruled at the rate of 

 500,000 lines to the inch, each line 

 drawn by the machine was iso^ooo of 

 an inch broad ! 



In reference to our previous article, 

 we do not mean to imply that a band 

 of 120,000 lines to the inch cannot 

 be resolved. The theoretical resolving 

 power of several objectives that have 

 been made exceeds 120,000 lines to 

 the inch, but we have never heard of 

 their actual limit of resolution. The 

 mere resolution of lines cannot be re- 

 garded as a test of the excellence of 

 an objective, and if such a band is 

 resolved by any objective now in use, 

 we would be inclined to regard it as 

 a strong indication that that objec- 

 tive is an inferior lens. This may 

 seem rather a surprising and bold as- 

 sertion ; but the resolution-tests that 

 have so longbeen relied upon as indica- 

 tive of the excellence of objectives, 

 have greatly retarded the progress of 

 microscopy in this country and in Eng- 

 land. They have led to the produc- 

 tion of a class of costly objectives, in 

 which general excellence has been 

 sacrificed to resolving power. A few 

 writers in England and America are 

 responsible for this unfortunate con- 

 dition of affairs. They have led a 

 great many microscopists in both 

 countries to place a false estimate 

 upon resolving power, and to under- 

 estimate other qualities that are of 

 equal importance. As a natural re- 

 sult, the makers of objectives have 

 followed the public demand, and pro- 

 duced such lenses as would satisfy it. 

 Only one of our prominent makers in 

 America has pursued a conservative 



