30 



THE AMEKICAN MONTHLY 



[February, 



ters with brilliant gems. Since the 

 molecules lie many deep, some can- 

 not get light at all, and they appear jet 

 black ; others in lower planes are 

 brighter. As they are found approach- 

 ing the I -90,000th of an inch in diame- 

 ter, the central focal lights disappear. 

 To produce the best effects, a con- 

 denser free from spherical aberration 

 is employed of about 55 degrees aper- 

 ture, acting axially with direct light 

 Some other still more wonderful ap- 

 pearances will be glanced at further 

 on, when transcendent definition is 

 approached. (Bright daylight — 

 even, I might say, dull daylight — is 

 'always preferable for the develop- 

 ment of superb definition) . 



Minute focal changes produce ap- 

 pearances of great interest, showing 

 the actions of light upon refracting 

 spherular bodies. 



Supposing, first, a low focal plane 

 be taken, there is a brilliant white 

 disc, surrounded with its jet black 

 annulus. If the corrections are very 

 carefully attended to, under correc- 

 tion causes it to turn crimson red, 

 with a fainter rim. A true correction 

 gives a bluish, or peacock blue spar- 

 kle in the bead. At the highest focal 

 plane, the emanating cone generally 

 produces an intensely black dot above 

 the spherule. These black dots are 

 seen often enough above refracting 

 molecules, scattered about insect 

 scales, especially those placed upon 

 the cross-bars connecting the ribs. 



Such elevated dots may be called 

 eidolic. If the spherule be large 

 enough, say i -20,000th in diameter, 

 this eidolic dot takes the form of a 

 small bead suspended above without 

 blackness, but faintly and delicately 

 shaded, so as to look almost planetary. 

 In observing these niceties, the 

 greatest care must be taken to reduce 

 the angular aperture of the condenser 

 as small as convenient for sufficient 

 illumination. There can be but little 

 doubt that a minute pinhole placed 

 over the condenser reduces the effect- 

 ive aperture of both objective and 

 condenser. 



Provisional Key to Classification of 

 Algae of Fresh Water.— VI. 



BY THE EDITOR. 



S^Continued from p. 2jj, vol. vi.'\ 



FaniilyY^. CEdogoniaceje. 



Filamentous algje living in water, 

 consisting of branched or unbranched 

 series of cells, with a basal cell. The 

 basal cell is usually obovate, or 

 swelled and lobate, often ending in a 

 disk-like attachment. 



Oogonia naked, in the vegetative 

 series. Antheridia filamentous, con- 

 sisting of few or many successive 

 cells. Spermatozoids spherical, 

 single or two in a mother-cell. 

 Oospores single in each oogonium, 

 formed of the entire contents, usually 

 red when ripe, producing swarm- 

 cells after long rest. 



Asexual propagation by swarm- 

 spores, formed singly in the vegeta- 

 tive cells out of the entire contents, 

 provided with cilia surrounding a 

 hyaline end. 



Synopsis of Geiiera. 



Filaments unbranched, when in 

 fruit with spherical, tumid cells. 



CEdogonhim^ 79- 



Filaments branched. Cells with 

 long bristles. Btilbochcete^ 80. 



79. Genus CEdogonimn Link. 



Unbranched. Antheridia pro- 

 duced either on the same filaments 

 with the oogonia (monoecious spe- 

 cies) or on special male filaments of 

 very different size and origin (di- 

 oecious species) . The male plants 

 may be short, one or several-celled 

 filaments, growing upon or near the 

 oogonia like epiphytic dwarfs (nan- 

 nandrous or dwarf males) , or they 

 may be in filaments, the male cells 

 interspersed among the vegetative 

 cells, resembling the female filament, 

 or often much smaller (macrandrous 

 males). The dwarf males arise 

 from male swarm-cells or andro- 

 spores. The androspores may be 

 produced in two ways : — 



I . In special abbreviated cells of 

 the female filament (gynandrosporous 

 species). 



