ON THE USE OF 



The angles so made by the dotted lines are measured by 

 a graduated semicircle of 180, under peculiar manage- 

 ment of light well known by opticians, but beyond our 

 present inquiry ; and denote the number of extreme lateral 

 rays which the object-glass admits. 



The larger the angle the greater are the number of rays 

 admitted and the more brilliantly the object is illuminated, 

 the greater, consequently, is the defining power. Experi- 

 ment has shown that obliquity of light is needful for the 

 perception of the most delicate markings, and that an out- 

 line visible with an object-glass of small angular aperture 

 admitting but few oblique rays, as in fig. 1, would be filled 

 up with lines of beauty, and striae of inconceivable delicacy 

 under an object-glass of large aperture, as fig. 3, which 

 gives it an oblique illumination. 



The markings on butterfly scales and the valves of Dia- 

 tomaceae will illustrate this. 



Again, it is asked, What is that particular fault which 

 object-glasses by inferior makers are liable to, called 

 spherical aberration ? It is when objects at the circum- 

 ference of the field are not in focus at the same time as those 

 in the centre, or when part of a single object fades away 

 towards the circumference. 



Another fault is chromatic aberration, when coloured 

 fringes surround the object under examination, whereas an 

 achromatic lens shows a clear colourless field and a purely 

 bright object. 



