464 



MANIPULATION. 



balanced, it becomes necessary to explain how this angle 

 may be measured with accuracy. 



Method of measuring the Angle of Aperture of Object-glasses. — 

 Various plans have been adopted from time to time to ascertain 

 this important point ; but by far the best of all is that proposed 

 by Mr. Lister, in his paper in the 121st volume of the Philo- 

 sophical Transactions, p. 191, which is as follows: — "Fix a 

 piece of paper on a table, and on it place the microscope, with 

 its body horizontal, and one of the eye-pieces on ; set a candle 

 on a level with it, a few yards distant ; then having directed 

 the body of the instrument so far on one side of the candle as 

 that the light from it shall bisect the field vertically, leaving 

 half of it dark, trace on the paper a line corresponding to the 

 side of one of the legs ; now, taking the focus of the object- 

 glass as a pivot, turn the microscope horizontally to the other 

 side of the candle till the opposite half of the field only is 

 illuminated, and mark again on the paper the position of the 

 side of the leg. The measure of the angle traversed, shown 

 by the two hues, is that of the pencil of light." The makers 

 of the object-glasses do not usually employ a microscope for 

 the purpose of measuring the angles, but an instrument of the 

 form represented by fig 260, for the copy of which the author 



Fig. 260. 



is indebted to Mr. Koss ; it consists of a piece of mahogany, 

 or other hard wood, of a semicircular figure, about haLf-an- 



