20 



ON PERSONAL EQUATION, WITH REFERENCE TO MICROSCOPY. 



in all others — the clearest view being at right angles to that which 

 is least distinct. The direction and amount of this defect can be 

 ascertained and corrected by the use of cylindrical lenses. The 

 existence of astigmatism can sometimes be inferred from the ob- 

 servations both of astronomers and microscopists, and it is pro- 

 bably very common, though in general unsuspected. 



Ihe foregoing notes are only intended to draw the attention of 

 microscopists to a subject which has not hitherto been considered 

 of importance. Its full discussion, if undertaken by some com- 

 petent authority, could not fail to be of great interest and utility. 



The Aquarium as a Field of Microscopical Research. 

 By T. Charters White, M.R.C.S., F.R.M.S, &c. 



{Read Jan. 22, 1875.) 



It is now nearly 25 years since the Aquarium, as it exists in its 

 present form, elaborated by the observations of Dalyell, Warring- 

 ton, Gosse, and Lloyd, became un fait . accompli : and although it 

 has been largely employed and productive of much good work by 

 many observers, it is to be regretted that it has not been more 

 freely used as an aid to microscopic research. 



While the last quarter of a century has seen much progress in 

 other departments of natural science, comparatively little has 

 been added to our knowledge of the development of the many 

 forms of animal and vegetable life with which the aquatic king- 

 dom abounds, and two causes exist which may account for this. 

 First, microscopists as a rule are too much satisfied with the 

 collection of pretty objects for their cabinets, and not sufficiently 

 alive to the interest and importance of observing and recording the 

 various stages through which those much admired objects attain 

 their ultimate condition ; and secondly, an idea seems prevalent 

 that the maintenance of an Aquarium in such a state as would 

 conduce to the growth and development of microscopic life is a 

 matter of immense difficulty, and only to be undertaken by a few 

 especially gifted individuals. No greater mistake can interpose 

 itself in the way of your intellectual enjoyment than this, for the 

 maintenance of an Aquarium is a much more easily managed affair 



