CONSTRUCTION OF THE MICROSCOPE. 



tion is wholly independent of any particular construction of the object- 

 glass, as in all cases where the object-glass is corrected for an object 

 uncovered, any covering of glass will create a different value of aberra- 

 tion to the first lens, which previously balanced the aberration result- 

 ing from the rest of the lenses ; and as this disturbance is effected at 

 the first refraction, it is independent of the other part of the combina- 

 tion. The visibility of the effect depends on the distance of the object 

 from the object-glass, the angle of the pencil transmitted, the focal 

 length of the combination, the thickness of the glass covering the 

 object, and the general perfection of the corrections for chromatism 

 and the oblique pencils. 



" With this adjusting object-glass, therefore, we can have the requi- 

 sites of the greatest possible distance between the object and object- 

 glass, an intense and sharply-defined image throughout the field, from 

 the large pencil transmitted, and the accurate correction of the aberra- 

 tions j also, by the adjustment, the means of preserving that correction 

 under all the varied circumstances in which it may be necessary to 

 place an object for the purpose of observation." 



Angle of Aperture. 



The definition of an object-glass much depends upon the increased 

 " angle of aperture." The angle of aperture is that angle which the 

 most extreme rays that are capable of being transmitted through the 

 object-glass make with the point of focus: b a b, in figs. 28 and 29, is 

 the angle of aperture ; but it will be seen that the angle of aperture is 



much greater in fig. 28 than in fig. 29, which 

 represents an uncorrected lens ; consequent- 

 ly a much larger quantity of light is trans- 

 mitted by the former than by the latter, 

 when any object is subjected to examina- 

 tion. In order to see an object at all dis- 

 tinctly with an uncorrected lens, it is ne- 

 cessary to diminish the aperture so much, 

 by the aid of stops, as to interfere with the 

 transmission of the amount of light required 

 to see the object perfectly. We shall have 

 occasion to speak of this again. 



A very perfect instrument for measur- 

 ing the angle of aperture, designed by Mr. Gillett, consists of two 

 microscopes, the optical axes of which may be adjusted to coincidence. 

 One of these is attached horizontally to the traversing arm of a hori- 



