TEST OBJECTS. 461 



of the window seen in the globule; when the globule is within 

 the focus of the object-glass, the light of the window will be 

 seen to swell out into a circular disc ; these appearances are 

 more or less accompanied by prismatic colours. It would be 

 in vain to attempt a description of all the changes that take 

 place, as the globule is brought either within or without the 

 focus ; these have, in some measure, been illustrated in one of 

 Dr. Goring's papers, published in the Microscopic Illustrations 

 of Mr. Pritchard, to which the author would beg to refer his 

 readers ; suffice it here to say, that when an achromatic com- 

 bination, that is perfectly corrected for spherical and chro- 

 matic aberrations, is employed, the globule should exhibit 

 similar appearances, both within and without the best focus; 

 and that when at the best focus, the point of light should be 

 seen as a minute disc, free from irradiations and colour, except 

 a general blueness, which results from the irrationality of the 

 spectra of the different glasses of which the object-glass is 

 composed. 



It would be needless to enter farther into this complicated 

 subject, as rarely, if ever, wiU the microscopist find it neces- 

 sary to have recourse to such delicate manipulation to try the 

 quality of his magnifying powers, more especially as the 

 subjects now employed as tests of their definition are of such 

 value and so manageable, that in some cases a simple inspec- 

 tion, by a practised eye, will at once determine the respective 

 merits of any achromatic combinations, as weU as the amount 

 of skUl and care displayed in their construction. 



Power of definition depends, in a great measure, upon the 

 angle of aperture of the object-glass, and correctness of defi- 

 nition upon the balance of the aberrations and the perfection 

 of the workmanship. As it is of the greatest importance that 

 the meaning of the term angular aperture should be well 

 understood, it has been deemed right in this place to enter into 

 an explanation of the same, as many persons unacquainted 

 with the subject are at a loss to conceive how more light can 

 pass through a combination of three pairs of lenses, than 

 through a single lens of equal magnifying power. 



Angle of Aperture. — The following description of this sub- 



