118 J. BURTON ON TWO INSTANCES OF 



in figure and in colour with the slightest alteration in focus^ 

 but are certainly almost colourless,] 



Take now the second case. Early in August I visited " The 

 Welsh Harp " reservoir at Hendon. This is a fairly large piece 

 of water, and of considerable depth. Here, all along the lee 

 shore, the water was turbid, dark green, and without any trans- 

 parency. With the naked eye one could see floating in it 

 quantities of minute bodies. Where blown by the wind into 

 lines or round floating objects, these bodies were seen to be the 

 cause of the colour. On a subsequent visit, as in the other 

 instance mentioned, these bodies were thrown on the shore in 

 masses, just as small seaweeds and other debris are left by the 

 receding tides. I have never seen duckweed or any of the 

 common algae thrown up by fresh water in such quantities. 

 This condition of things continued longer than in the case of 

 the Totteridge pond, and there was abundance of the organism to 

 be seen late in September ; but on a visit in October I could find 

 none along the margin of either part of the reservoir. To my 

 sui^prise, this organism, when examined under the microscope, 

 proved to be almost identical in size and structure with the other, 

 diflfering only in colour, which was a bluish green. It had the 

 same refractive bodies, which were equally difficult to observe 

 satisfactorily. Perhaps the filaments varied somewhat more in 

 size than the others, measuring from about 3 to 5 /a. Again I 

 could not identify the species : in no book, to my knowledge, 

 is there a figure or description resembling it. Some living 

 specimens were sent to an authority on algae, and he, with con- 

 siderable reservation, pronounced it to be Oscillatoria ayardhii. 

 That still leaves the clay-coloured one to be identified; and on 

 that point I have a theory to offer, but it is not supported by any 

 authority. Practically the only difference between the two is in 

 the colour. The green one, when dried on the side of a glass, is 

 of a bright blue-green ; the yellow one scarcely changes its hue 

 at all : there is no indication of the presence of chlorophyll or 



