298 HOW TO SEE WITH THE MICROSCOPE. 



WORK OVER DRY MOUNTS WITH HIGH APERTURES OBJECT- 

 IVES. 



After, and not before, the student has become profi- 

 cient in the preceding lessons, it will be advantageous 

 for him to procure dry mounts of the acknowledged 

 diatom tests, and study well their peculiar action under 

 the objective. At a glance it will be noticed that the 

 dry frustules are much more vigorous than those mounted 

 in balsam. They have more body, appear more solid,, 

 the image is stronger every way. Hence it is that an 

 objective may shew a dry mount tolerably well, and yet 

 be utterly defeated by the same valves when balsam 

 mounted. Should the student be well versed in our pre- 

 vious instructions, he will find little difficulty in dealing 

 with dry mounts. As a rule, the Litter are to be illu- 

 minated with pencils of less obliquity than objects 

 mounted in balsam. In the dry mount, too, we have 

 greater contrasts of light and shade. These differences 

 will be at once noticed by the intelligent pupil, who 

 will find little difficulty in adapting himself to the situ- 

 ation. He should keep in remembrance this one fact,, 

 to wit: That, owing to the superior brilliancy pert-lin- 

 ing to the dry mount, little differences in the collar ad- 

 justment are not so perceptible as is the case with bal- 

 samed objects; nevertheless, the difference is there, and 

 well worth the study necessary to recognize these little 

 differences. Again, should the learner desire to com- 

 pare one objective with another as to their comparative 



