INTRODUCTORY RKMARKS. 37 



name of Engiscope* for the instrument we are now 

 considering. 



In this instrument, it is of the utmost importance that 

 the object-glass forming the image, which is to undergo 

 a second amplification by the eye-glass before it meets 

 the eye, should be as perfect as it can possibly be made ; 

 becauge, every error occasioned by it will be magnified 

 by the eye-glass, and thus cause very great confusion. 

 Dr. Goring having this in view, directed the late Mr. 

 William Tulley, in 1824, to make him a triple achromatic 

 object-glass, for a compound microscope. With this, 

 after many trials, during which the Doctor had contrived 

 to enlarge the aperture of his lens, he found that he 

 could distinguish some delicate markings on certain 

 animal tissues, which, with equal magnifying powers 

 having less aperture, he had not been able to discern. 



Hence originated the discovery, that the penetration 

 of a microscope is dependent upon the angle of aperture 

 of its object-glass. 



As this discovery has tended, in a great measure, to 

 bring the microscope to a high state of perfection, I may 

 be permitted to give a familiar explanation of the reason 

 why an increase of aperture is so efficacious. For the 

 details of the subject, the reader is referred to Dr. 

 Goring's own account, in the Micrographia, Chap. VII. 

 I am the more anxious to give this explanation, because 

 many persons are at a loss to conceive how it is that in 

 a compound achromatic microscope, where the light has 



* See Chapter VII. 



