182 HOW TO SEE WITH THE MICROSCOPE. 



up the rays from the radiant at the distance of the focus 

 of this supplementary lens. 



" The plate, P, is graduated on its circular edge, as in 

 the figure, to two degrees, and the arm, A, has a swing 

 of seventy degrees of arc each way from the axis of the 

 microscope. An index-line is marked on the bevelled 

 edge of the carriage 10 from the axis of the condenser, 

 which must be added to or subtracted from the real ob- 

 liquity of the illuminating rays. 



" It is obvious that any observation made and duly re- 

 corded as to its conditions, as of obliquity of incidence 

 of illuminating pencil or ray, form of the pencil or beam, 

 focal length and distance of the condenser, such obser- 

 vation could be successfully repeated. The record of 

 the obliquity of the most oblique rays reaching the ob- 

 ject directly, and giving view of it at the eye-piece with 

 luminous field, would express the 'balsam' aperture, 

 or more correctly, the half interior aperture of the ob- 

 jective when the front lens of the objective and the 

 traverse system are of glass of similar refraction. 



" Having thus the ' balsam ' angle, we readily calcu- 

 late or learn the corresponding angle for glycerine, 

 or water, or any medium of which we have the index 

 of refraction. A corresponding notation, perhaps for 

 air, might be engraved in juxtaposition on the basilar 

 plate." 



