On the Single Vision Prism of Calcareous Spar. 333 



of the sides also five-tenths of an inch. With the exception 

 of a very small segment towards the outer or blue edge of 

 the extraordinary ray, the whole disc is visible, at least with 

 a slight inclination of the eye. In placing the prism above 

 the lens, some nicety is required, in oi'der to obtain the 

 greatest possible field of view. It requires to be inclined a 

 little to the axis of the microscope. If the acute solid angle 

 of the prism be farthest from the body of the observer, the in- 

 clination must be towards the body. 



In constructing instruments of this kind, the greatest accu- 

 racy is necessary. The coiTesponding angles of the two por- 

 tions must be equal. Both the terminal and sectional planes 

 must be set on so as not to incline in the least perceptible 

 degree more to one side than the other. When inaccuracies 

 in these respects occur, the circular rings are sure to be de- 

 formed. It may, therefore, be of use to point out an easy 

 method of determining whether the construction be accurate 

 or not. At the distance of two or three yards, direct the 

 prism to the crossing of the astragals of a window, make the 

 prism revolve slightly backwards and forwards, and if no 

 motion of the astragals ensue, the terminal planes may be con- 

 sidered as parallel. Incline it, then, until one of the astragals 

 coincide with the junction of the two rays, and if there be no 

 overlapping of the astragal, or if it appear as distinct and 

 defined as when seen with the naked eye, the instrument may 

 be considered as having been properly constructed. 



On the Geographical Distribution of Insects. (Continued 

 from p. 111.) 



Stations. — When we observe the great number of localities 

 in which certain species of insects are foimd, we might be 

 tempted to believe that these animals have no well determin- 

 ed stations. It is true, that theirs are less fixed than those 

 of vegetables, and that it would be difiicult to assign a precise 

 one to some species ; but, in general, these are exceptions, 

 which a little observation enables us to recognise as such. 



l^et us take, for example, a very natural genus, admitted 



VOL. XXVII, NO. LIV. OCTOBER 183D. Z 



