Editorial. 155 



[The young plant is attached to foreign bodies, and consists of a 

 cylindrical cell in which are produced 8 gonidia ; the top of the cylin- 

 der falling off the gonidia emerge and form an umbel of similar 

 cylinders, the bases of which stick in the primary cell. The gonidia of 

 the third generation are set completely free and become primary cells 

 of new families {Mic. Dzc/.)^^ 



Sc. arbtiscida, A. Br. Cells 0.2-0.3 lo^ig. In ditches, swamps, etc. 

 5. Gen. Hydrodictyon, Roth. Cells cylindrical, with the ends 

 bound together into large reticulated hollow sacs. The 

 ordinary swarm-spores (Macrogonidia) arrange themselves 

 into new sacs within the mother cells, which later disap- 

 pear ; others (Microgonidia) swarm out, remain first at rest, 

 then return, after a generative change, to their original 

 form. 



i£(litoviaL 



MICROSCOPIC VISION. 



The subject of microscopic vision has been carefully studied 

 by Prof. Abbe, but his results have not yet been placed before 

 the American public in a clear light. Recently. Mr. Frank 

 Crisp read a paper before the Quekett Club entitled, "The In- 

 fluence of Diffraction in Microscopic Vision," in which the 

 main results of Prof. Abbe's studies, upon the formation of 

 the image in the microscope, are plainly stated. On removing 

 the eye-piece of the microscope, previously focussed upon a 

 lined object, and looking down the tube, we see not only a 

 small circle of light but also a series of colored ovals with 

 the blue nearest the center. These ovals are diffraction images 

 of the open front of the objective. As the light passes through 

 the lined object it is decomposed and the colored rays pro- 

 duce the colored images of the opening, the blue being with- 

 in the others, on account of its greater refrangibility. 



From a study of these diffraction images Prof. Abbe has 

 adduced the theory that the image formed in the microscope 

 is generally made up of two images, one of which shows the 

 larger parts — the outlines &c.; the other giving the finer struct- 

 ure — markings and minute structure. The first is the dioptric 

 image, formed as we are accustomed to explain it; the other 

 is produced by the spectral rays. If we stop out the central 

 rays so that only the spectra arc to be seen, and then replace 

 the eyepiece, we shall only see the fine markings. If we stop 



