Miscellaneous. 487 



ascertain the influence of these upon the limits of vision. Thus we 

 may use as an object a very thin glass capillary tube placed in water, 

 and compare it with tender organic tubes and vessels, which may also 

 be seen in water, but whose limit of visibility is of course far more 

 circumscribed than that of absolutely opake objects. 



In fact this method admits of innumerable variations, and is con- 

 sequently of most extensive apphcation. Besides, when proper pre- 

 cautions are taken, it gives results perfectly sure and comparable. 

 Especial care is, however, requisite in the mode of illumination. For 

 it is certain, that when the field has a clear white ground, the contrast 

 causes minute opake bodies {i. e. objects which are dark by trans- 

 mitted light) to continue visible, which against a grayish or light-blue 

 back-ground could not be seen. Hence it is by no means indifferent 

 to receive on the mirror light from a white cloud, from a dull over- 

 cast, or clear blue sky. Artificial light cannot be used in these ex- 

 periments, for the image of the flame becomes diminished like the 

 object, and hence a clear field of view is not to be obtained. The 

 observations must consequently be made by daylight ; and whenever 

 comparable results are sought for, the mirror should always be directed 

 to the clear, blue, cloudless sky — this being a distinct atmospheric 

 condition to which others in similar circumstances may refer in con- 

 ducting the same experiment. The mode of ascertaining the limit 

 of vision, with a given amount of illumination, may be gathered from 

 different examples in the body of this work*. It will likewise be 

 found that for all such observations, even when the highest magni- 

 fying powers are employed, the J?at mirror is perfectly sufiicient, 

 since in the image in the field of view formed by the air-bell, all the 

 rays proceeding from the mirror are united and constitute an object 

 of considerable luminous intensity. — Monthly Journal of Medical 

 Science, April 1853. 



MARINE VIVARIA. 



To the Editors of the Annals 0/ Natural History. 



Weymouth, May 24, 1853. 

 Gentlemen, — With reference to plants for Marine Vivaria I 

 have to state, that some species, if not all, do equally well without 

 their roots being attached. On the 4th of May I placed a few mol- 

 lusks in a glass 8 inches in height and 4 across ; I also placed in it a 

 plant of Rhodymenia palmata, but which had no root attached ; I 

 therefore attached it by means of a thread to a small piece of stone 

 in order to keep the plant erect. This plant alone has supphed the 

 water with oxygen to this day, and appears as healthy as the day it 

 was put in, now three weeks since. The animals are alive and the 

 water has not been changed. 



I am. Gentlemen, yours obediently, 



"William Thompson. 



* It is unnecessary to introduce any examples here, the author's descrip- 

 tion of his method being both Ml and suggestive. — Editor. 



