PRACTICAL HISTOLOGY. 



work with. It may be safely stated that for ordinary histo- 

 logical purposes, lenses of "small," or rather of medium, 

 are preferable to those of " large " angle. In such a lens 

 as -J- inch, 70 is considered a "small," and 130 a "large " 

 angle. 



38. Adjustment of the Objective for covered and 

 uncovered Objects. If rays pass from a luminous 

 point (S, Fig. 27) to the eye obliquely through a parallel 



glass plate, there is an 

 apparent displacement 

 of the point to the 

 position S'. The dis- 

 tortion of an oar when 

 dipped in the water is 

 due to a similar cause. 

 If, therefore, a plate of 

 glass be interposed be- 

 tween the lens and an 

 object, the latter is dis- 

 torted, and the amount 

 of distortion increases 



with the thickness of the glass. Thus, in Fig. 28, we have 

 rays diverging from the point a and passing through a 

 cover-glass c. The 

 ray r, close to the 

 perpendicular/, would 

 appear to an observer 

 to proceed from an ob- 

 ject very nearly in the 

 position of a. the ray 

 r from an object at 

 a, and r" from an 

 object at a". The 

 single luminous point 

 a would thus be spread 

 out and rendered indistinct ; and the more so the thicker the 

 cover-glass ; hence the importance of employing thin cover-glass. 

 It is evident from Fig. 28 that the divergence of the rays is 

 increased by the cover-glass ; hence, if the objective be ad- 



FIG. 27. Apparent displacement of a luminous 

 point by a parallel plate. (Deschanel. ) 



FIG. 28. Effect of the cover-glass. 



