Objects JOT the Microscope. 5 



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 is 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 stria? 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- 

 tom acea? 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. 



