162 SELECTION AND USE 



or three inches focus, mounted on the arm, A, will conveniently 

 take 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 sub- 

 tracted from the real obliquity of the illuminating rays. 



It is obvious that any observation made and duly recorded as 

 to its conditions, as of obliquity of incidence of illuminating 

 pencil or ray, form of the pencil or beam, focal length and dis- 

 tance of the condenser, such observation could be successfully 

 repeated. The record of the obliquity of the most oblique rays 

 reaching the object directly and giving view of it at the eye- 

 piece with luminous field, would express the "balsam" aper- 

 ture, or more correctly, the half interior aperture of the objec- 

 tive when the front lens of the objective and the traverse system 

 are of glass of similar refraction. 



Having thus the "balsam" angle, we readily calculate or 

 learn the corresponding angle for glycerine or water, or any 

 medium of which we have the index of refraction. A corres- 

 ponding notation, perhaps for air, might be engraved in juxta- 

 position on the basilar plate. 



The Spot Lens. This is a plano-convex lens of very high 

 curvature (it is generally hemispherical), so mounted that its 

 distance from the object may be adjusted in such a way that 

 the rays which pass through it, may be brought to a focus 

 on the object. The central jrays are stopped out by means of a 

 black spot (hence the name), so that the object is illuminated 

 wholly by rays which are of too great obliquity to enter the 

 object-glass, except when their direction is changed by the 

 object. The latter, therefore, appears brilliantly illuminated 

 on a dark ground, and in many cases features which could not 

 otherwise be seen are shown very distinctly. 



The larger diatoms, insect scales, and similar objects are 

 peculiarly suited to this method of illumination, and some of 

 them are very beautiful as "'show " objects. 



