44 CONSTRUCTION OF THE MICROSCOPE. 



of definition, and the fine screw movement of the micro- 

 scope for correct focus. 



" It is hardly necessary to observe, that the necessity 

 for this correction 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 aber- 

 ration resulting from the rest of the lenses ; and as this 

 disturbance is effected at the first refraction, it is inde- 

 pendent of the other part of the combination. The visibi- 

 lity 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 cor- 

 rections of chromatism and the oblique pencils. 



"With this adjusting object-glass, therefore, we can 

 have the requisites 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 aberrations ; also, by the adjustment, the means of 

 preserving that correction under all the varied circum- 

 stances 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 in- 

 creased " angle of aperture." 

 The angle of aperture is that 

 angle, which the most ex- 

 treme rays that are capable 

 of being transmitted through 

 the object-glass make at the 

 point of focus : b a b, in 

 figs. 29 and 30, is the angle 

 of aperture ; but it will be 

 seen that the angle of aper- 

 ture is much greater in fig. 

 29 than in fig. 30, which re- 

 presents an uncorrected lens; 

 consequently, a much larger quantity of light is trans- 



