170 



THE AMERICAN MONTHLY 



[September, 



A perfectly sharp negative will 

 bear considerable enlarging without 

 noticeable loss of detail. In an arti- 

 cle published several months since* re- 

 ference was made to some negatives 

 about the size of an English six-pence, 

 of bacteria, which competent judges 

 pronounced excellent. These bore 

 enlarging to the size of ordinary lan- 

 tern positives. These were taken 

 without the eye-piece, in a very small 

 camera attached to the tube of the 

 microscope. Tjie opinion was ex- 

 pressed that ' better negatives of bac- 

 teria and very minute objects can be 

 produced without the eye-piece, by 

 obtaining more perfect small nega- 

 tives, than by original large nega- 

 tives.' Dr. Van Heurck also, as re- 

 corded in these columns, holds to the 

 same opinion. Nevertheless, one 

 may carry a very reasonable opinion 

 to such an extreme as to lose the value 

 of it. There is no reason for making 

 negatives so small that a lens is re- 

 quired to examine them. There is, 

 in fact, a decided disadvantage, in 

 that greater enlargement is required 

 by a photographic copying lens, and 

 however excellent such a lens may be, 

 it will not hold its sharp definition 

 when required to magnify. There- 

 fore, it is advisable to make the nega- 

 tive from the microscope at least as 

 large as a 3^^ by 4^; plate will take. 



Summing up this matter, we are 

 personally inclined to favor the use 

 of large plates, 8 by 10 inches for 

 example, in photomicrography, using 

 the lens with an amplifier instead of 

 an eye-piece, for the reason that large 

 pictures, highly magnified, can thus 

 be obtained of exquisite definition. 

 These will bear further enlarging 

 with the solar camera. There re- 

 mains, however, the consideration of 

 expense, and the inconvenience of 

 using such a large apparatus under 

 ordinary circumstances. It is un- 

 questionably more convenient, in 

 most cases, to use smaller plates, and 

 to work with an eye-piece. Still 

 better, to use an amplifier in place of 



*This journal, vol. vi, p. 28. 



the ocular, for then it is possible to 

 attach the amplifier to the camera in 

 such a position that when the object 

 is focussed with the eye-piece, it is 

 also in focus on the ground glass of 

 the camera when the latter is attached. 

 With such an arrangement a quarter- 

 plate camera can be used with per- 

 fect satisfaction, giving negatives 

 equal to any that can be made. 



The same cannot be said when the 

 ocular is used, although there is no 

 doubt thoroughly satisfactory results 

 can be obtained with the ocular on 

 small plates, as already explained. 



Provisional Key to Classification of 

 Alg* of Fresh Water.— IT. 



BY THE EDITOR. 



III. ORDER CONFERVOIDEyE Kirch- 

 ner. 



Multicellular, filamentous algas, 

 simple or bi-anched, rarely spreading 

 flat ; cell contents green [except in 

 Chroolepus\^ o\\\y rarely showing 

 a definite arrangement, but usually 

 an outer layer of colored plasma, or 

 the entire plasma uniformly colored ; 

 cell-wall never siliceous. 



It is probable that sexual propaga- 

 tion will be found universal in this 

 order, but at present it has not been 

 observed in all genera. Sexual pro- 

 pagation takes place by female cells 

 (oogonia) which contain usually a 

 single oosphere, rarely several, and 

 male cells (antheridia) in which 

 spermatozoids are produced. 



Asexual propagation by swarm 

 spores. 



[The unbranchedConfervoideas are 

 readily distinguished from the fila- 

 mentous Zygosporeas by the uniform 

 coloring of the cell contents in the 

 former and the characteristic distribu- 

 tion of the color in the latter. 



From what has been said above, it 

 need scarcely be added that the divi- 

 sion into sexual and asexual forms is 

 merely tentave, for convenience.] 

 a. Synzoospore^ and Asexual 

 Forms. 

 Sexual fructification by spermato- 



