186 HOW TO SEE WITH THE MICROSCOPE. 



SUNLIGHT. 



In the study of very minute and delicate structures 

 requiring the utmost separating or resolving- power of 

 the objective, remarkable effects are to be secured by 

 condensing sunlight on top of the object by means of 

 the concave mirror, the object being mounted with a 

 cover in the usual way. The objective used should of 

 course have wide aperture. The mirror being posed 

 slightly above the level of the stage, the sunlight is 

 thrown on the surface of the cover, and making a very 

 acute angle therewith. Although not absolutely neces- 

 sary for this purpose, those stands furnished with swing- 

 ing sub-stages, allowing the mirror to rise above the 

 level of the stage, are extremely handy and convenient. 

 By the employment of this illumination in conjunction 

 with object-glasses of wide angles, the most difficult 

 diatoms, such as amphipleurapellucida^frustulia saxonica+ 

 etc., are easily and forcibly displayed. The advantages 

 attending the use of monochromatic sunlight, as ob- 

 tained by the intervention of the cupro-ammonia cell, 

 or a plate of blue glass, have long been known. Thi& 

 illumination is procured most easily as follows: Cut 

 with a diamond, or the point of a file, a small piece of 

 the blue glass roughly to fit the cap of the eye-piece, 

 so that when the cap is restored to its place the blue 

 glass shall be between the eye and the eye-lens of the 

 eye-piece, and the light is thus modified before it reaches- 

 the eye. This is the handiest method of obtaining mo- 



