right half at the front or back of the objective is 

 covered with black paper or tinfoil so that only the 

 other or left half remains optically effective ; take for an 

 object a line or dot of light which can be easily pro- 

 duced by blackening one surface of a slide by smoking 

 in an oil flame and drawing a line across it with a point. 



Under-correction shows when the image has on 

 the right side a violet or blue border and on the left 

 a red or orange colored border. 



Over-correction shows on the other hand when the 

 left side appears violet or blue and the right red or 

 orange. 



Spherical Aberration and Cover Glass. As 



has been stated, there is a residue of chromatic and 

 spherical aberration in all ordinary achromatic lenses 

 and objectives. The use of cover glass influences the 

 spherical correction and while not appreciable to any 

 extent in low powers, it is very sensible in the medium 

 and high powers. Referring to Fig. 37 it will be 

 remembered that the rays from the object do not 

 reach the surface of the objective front uninterruptedly 

 but are changed in their course. If we make use of 

 the same illustration and extend the refracted rays 

 downward as shown by the dotted lines d e, f c, f c, 

 d' e until they meet at the axis, these points will be the 

 apparent location of the object and will appear to meet 

 in the planes e a and c b instead of at o. To neutralize 



