more than half the light is reflected from the front surface, without entering the lens at all, and yet more at higher obliquities. I do not mean to say that there is not a gain worth striving for in passing from-say 160° to 170°, but an objective, whose real air angle is more than that last named, will have an inconveniently short working distance. I intend to investigate the cause of this enlarged angle of dry objectives observed in the Abbe apertometer fashion, with the bull's eye. I find that the Spencer Student 4th inch, which measures 100° with the glass slide and lines, or by looking through the eye-hole at b, as above described, gives, by the latter mode, when the bull's eye is interposed, 63° as angle in glass, from which the computed air angle will be near 104°. Using the bull's eye, slide, and lines as directed, and looking through b the glass angle is 61°.5, from which the computed air angle is 100°. With objectives of shorter focal strength the agreement is closer. I do not yet see any need of adopting Dr. Abbe's notation, which is, indeed, strictly correct in theory, but it would be very inconvenient for many microscopists, as it necessitates the use of tables that some might not have, or others understand. The fact is, no objective can be made to do equally as good work dry and immersion; it should be either dry or immersion simply. Why not mark the angle just what it is? If the objective is an immersion of 116°, everybody will understand it, and none but a simpleton would confound it with 116° marked on a dry lens, or suppose that it meant an objective of the same resolving power, or, indeed, excellence anyway. Knowing that 82° on an immersion would be equivalent to 180° on a dry, everyone would soon learn the comparative values of the same angles marked on each, and neither would ever come dangerously near 180°. To compute rigidly the balsam or glass angle from the observed air angle, or vice versa, the objective must not only be acccurately focused, but the intersecting lines must be used; otherwise an exaggerated angle (varying very much in different objectives) to the extent of two to twelve degrees, may be obtained as the glass angle; and if, from this, we should compute the air angle, it might be ten or twenty degrees too much. Carefully used, the instrument will give entirely concordant results. Moving to the right, the point of intersection will travel in the same direction, as from Fig. 3 to Fig. 5, and to the left the change will be from Fig. 3 to Fig. 4; this is contrary to what one might at first suppose; a little reflection will show the reason for this. GENEVA. N. Y. DUBIOUS CHARACTER OF SOME OF THE GENERA OF FRESH WATER ALGÆ. BY REV. FRANCIS WOLLE. (Received February 24th, 1879.) Algologists have made a number of genera of unicellular plants, as Glaocapsa, Microcystis, Glæothece, Protococcus, and the like. My observations of these forms during the past few years induce me to question the place given them as plants, and to suggest that they are merely forms of gonidia or spores, or sporangia, various stages of development in the life history of filamentous plants. It has been observed by some authors, that forms of Sirosiphon are developed from cells similar to Glaocapsa cells, and that the two kinds of plants live together, also that the articles of the internal cellular structure of Sirosiphon filaments are often very similar to the cells of Glaocapsa, but that the one originates the other, and is in turn again reproduced by it, does not appear to have been entertained. This is not surprising, because each form appears to have a life of its own. The filaments develop and multiply, and the spores or gonidia develop and multiply; the latter not unlike some of the lower forms of animal life, as certain Infusoria. In the annexed plate I illustrate three genera of plants, and show how the plants are developed from spores, and how the spores are produced from the plants; and again, how spores reproduce spores often through three or more cycles. These spores represent as many different genera of the so-called unicellular plants. The changes continue through many generations, and sometimes the cells spread over extended surfaces before any of them prove fertile in reproducing the mother plant. As an illustration, I represent in Plate XVIII., Fig. 1, first a fragment of an old filament (A A) of a common Sirosiphon (alpinus). The cells of the plant are usually subspherical and lamellate as in the end (B) of the figure. At the other end they have undergone a change, both in color and in feature, from brown to olive-green and æruginous. These cells are filled, primarily, with a homogeneous endochrome; in this are formed very minute granules, the microgonidia. The cells slide out from the broken or decayed end of the filament (dd); then the microgonidia enlarge and the cells assume the character of sporangia, or spore bearers (eeee). Next, the microgonidia are seen. to sheath (ff); they enlarge still more (gg) and divide (h). Up to this period they are enclosed in an epidermis, or membranous tegument, which now breaks, and the enclosed cells are scattered (ii); these in turn also grow larger and larger, the internal cells divide, increase, and develop (klm), often repeating this process many times; at last, from one of the latter form (m), Glæocapsa, here and there the young filaments of Sirosiphon (no) are reproduced. In this process we have a number of different forms of the dubious unicellular plants which would be respectively classed as Microcystis (e e e), Glaocapsa (ffggln), Glæocystis (h), and Glæothece (ii). These are often found in masses, sometimes the one and sometimes the other predominating. Under certain conditions the mother plant will be produced, and under other conditions only the spores will be repeatedly developed. In a deep mountain ravine in this vicinity, where the two forms abound, the more exposed rocks are covered with growths of Sirosiphon and a few of the Glaocapsa cells, while on other cliffs, which are shaded and dripping with moisture, the so-called Gleocapsa forms are the most abundant, with only occasional filaments of Sirosiphon, and may be drawn off by handfulls. They are evidently merely different conditions of the same plant. Fig. 2 represents another form of Sirosiphon, found by Mr. Brandegee at a soda spring in Colorado. The specimen was scant, but the forms were very distinct, especially in the spores. a is a sporangium, a Microcystis form, and bbb the Glaocapsa DESCRIPTION OF PLATE XVIII. Fig. 1, Reproduction of Sirosiphon Alpinus. Fig. 2. Sirosiphon, n. sp.? |