HINTS TO MICROSCOPISTS. 73 



tion. This point of transition (the circumference of the yellow spot) 

 was not circular, but the marrow-like globules radiated rather farther in 

 a star-like form at single points, which were not situated at regularly 

 recurring distances. A complete measurement I was unable to effect. 



I had the opportunity of submitting this observation to the Physician 

 General, Dr. Linden, who expressed himself perfectly assured of its 

 correctness. It appears to me to be so far important that it affords the 

 means of a simple and mechanical explanation of the phenomenon, that 

 a portion only of the retina, namely, that which answers to the poste- 

 rior part of the axis of the eye, possesses the power of distinct vision. 



to 



II. ON THE CULTIVATION OF VALISNEUIA, CHARA, ETC., FOR 



MICROSCOPIC PURPOSES.* 



By Cornelius Varley, Esq. 



I OFFER the following remarks as the result of my experience to those 

 microscopists desirous of preserving the different Charas, Valisneria, 

 and Hydrocharis, or Frog bit ; in all of which the circulation may be 

 well observed. 



In cultivating these plants, it is only requisite to take notice of the 

 circumstances under which Chara naturally thrives, and to imitate them 

 as nearly as practicable. 



Firstly. The Chara tribe is most abundant in still waters or ponds 

 that never become quite dry ; if found in running water, it is mostly 

 met with out of the current, in holes or side bays, where the stream has 

 little effect, and never on any prominence exposed to the current. If 

 the Chara could bear a current, its fruit would mostly be carried on and 

 be deposited in holes ; but it sends out from its various joints very fine 

 long roots into the water, and these would by agitation be destroyed, 

 and then the plant decays ; for although it may grow long before roots 

 are formed, yet when they are produced, their destruction involves the 

 death of the plant. In order, therefore, to preserve Chara, every care 

 must be taken to imitate the stillness of the water, by never shaking or 

 suddenly turning the vessel. It is also important that the Chara should 

 be disturbed as little as possible, and, if requisite, it must be done in the 



* In a letter to the Editor. 



