50 



THE AMERICAN MONTHLY 



[March, 



board is the Kiibel heliostat. The 

 whole apparatus rests upon a solid 

 table-case, with drawers for accesso- 

 ries. When a photograph is to be 

 made the window^ is opened and the 

 base-board pushed forward until the 

 heliostat is outside in position. The 

 window is then closed, and the ap- 

 paratus is ready for use. 



The light from the heliostat is re- 

 ceived upon a convex lens of con- 

 venient focal length — the one we use 

 is two inches in diameter and has a fo- 

 cus of twelve inches — and may be di- 

 rected upon the object either with or 

 without the mediation of a substage 

 condenser. The light should not be 

 focussed upon the object, however, 

 owing to the heat, which would be 

 likely to injure the mount. A short 

 distance either side of the focus the 

 heat v^nll not be sufficient to do 

 harm. 



A cell with parallel glass sides, 

 containing a solution of ammonio 

 sulphate of copper should be inter- 

 posed between the lens and the ob- 

 ject, to give a powerfully actinic blue 

 light without the glare of strong sun- 

 light. The cell should be about half 

 an inch in thickness. The solution 

 is made by dissolving blue vitriol in 

 water and adding ammonia until the " 

 precipitate which forms at first is re- 

 dissolved. It is usual to focus the 

 object with bright sunlight, then to 

 interpose the blue cell and make the 

 exposure. 



In using the electric light, it will 

 be found most convenient to employ 

 an incandescent lamp, although the 

 arc light is much more powerful. 

 The incandescent light is more 

 steady, more easily managed, and 

 quite as satisfactoiy. To ti'eat this 

 part of the subject in full, however, 

 would require more space than can 

 be given at this time. We will only 

 add that several manufacturers have 

 introduced lamps well adapted to this 

 work, and no special instructions are 

 necessary. 



In using lamp-light the best form 

 of burner is one having a very broad, 



flat wick, the edge of which should 

 be directed toward the object. A 

 wick three or four inches broad is 

 desirable. Such lamps as are used 

 in the best stereopticons give an ex- 

 cellent light for this purpose. The 

 light should be condensed upon the 

 object by suitable lenses. With a 

 common bull's-eye lens and an Abbe 

 condenser in the substage, excellent 

 work with powers as high as a ^ can 

 be done, using the light of an ordi- 

 navy hand lamp. 



Next month we shall describe the 

 process of taking the picture. 

 [7^(? de conttnued.'\ 

 O 



Provisional Key to Classification of 

 Alg« of Fresh Water.— YII. 



BY THE EDITOR. 



'^Continued frotn p. J/.] 

 I^a/nily X. Coleoch,«;taCe^. 



Small disk-like families, light 

 green, forming a flat or cushion-like 

 parenchymatous thallus ; cells oblong 

 or expanded in front, sometimes 

 bearing long, colorless bristles on the 

 back or upper surface. 



The oogonium is a single cell at the 

 end of a vegetative series. The an- 

 theridia give rise to a single spermat- 

 ozoid in each, which are set free and 

 move by the aid of two cilia. The 

 fertilized oogonia become enclosed 

 in a protecting coat, rest through 

 the winter, and in the spring the con- 

 tents divide, giving rise to several 

 swarm-cells, which escape and grow 

 into new plants. 



Asexual reproduction by means of 

 swarm-spores, formed within any of 

 the vegetative cells, with two cilia. 



8i. Genus Coleochcete Brebisson. 



Thallus pale green, forming small 

 disk-like grow^ths on other algse, dead 

 plants, etc., consisting of series of 

 radially disposed cells, often laterally 

 connected. Some of the cells bear 

 long, hyaline hairs. 



[This is a very common genus, 

 representatives of which can b«e 

 found almost always growing on the 



