MICROSCOPIC ILLUSTRATIONS. 



The figure of concave lenses may be tried upon the 

 same principle as metals, for they will reflect enough light 

 from a real or artificial star to allow us to judge of the 

 nature of their curves. When a plain metal operates 

 along with a concave one, as in the case of the Amician 

 reflecting engiscope, an irregularity in the outline or con- 

 tour of a disc out of focus is sometimes generated by an 

 imperfection in the surface of the former, which may be 

 known by trying the concave on a lucid point by itself, 

 when, if the plain one only is in fault, of course the irre- 

 gularity of the disc will disappear. The state of a plane 

 reflecting surface as to exactitude may be known by ob- 

 serving the manner in which it reflects the image of some 

 regular-shaped body (a right line is as good as anything), 

 placed at a distance from it, the eye being also considerably 

 removed from the metal ; and still better by adjusting very 

 exactly the focus of a telescope upon some distant body, and 

 then reflecting an image of it by means of the plane metal 

 to be tried (which is to be placed at some distance from 

 the telescope), and viewing the same object again in the 

 telescope, when, if the plane metal is perfect in point of 

 figure, no difference of adjustment will be required to pro- 

 duce distinctness. 



IMPERFECTIONS IN CONCAVES OF FLINT GLASS, &c. 



These rarely occur in the small lenses used for the object- 

 glasses of engiscopes ; they may be seen when they exist, 

 by adjusting the instrument upon some lucid point, and 

 then removing the eye-piece and looking at the object- 

 glass with the naked eye, when the strise in it will become 



