EMPLOYMENT OF COVER-GLASSES. 271 



microscope-lamps provided with a large condensing lens are 

 mentioned. The same result may be attained with an ordinary 

 moderator or petroleum lamp by placing before the flame a 

 globular vase filled with a weak solution of ammonio-oxide of 

 copper. In the opinion of competent judges, the light thus 

 obtained is agreeable to the eye and at least equal to average 

 cloud-light. 1 



2. THE SELECTION OF THE MAGNIFYING POWER. 



The preparation to be investigated is first viewed with a mode- 

 rate amplification, because a better survey of the material present 

 is thus obtained, and the best part of the sections, or the most 

 favourable objects, may be conveniently selected for more detailed 

 observation. If the lowest-power objective at disposal magnifies 

 too much, a further diminution of the image may be obtained by 

 shortening the body-tube, or by removing the anterior lenses of 

 the objective. The sharpness of the image suffers materially in 

 the latter case, unless the objective is specially constructed. 



In proceeding to higher amplifications, we must remember that 

 the optical power is chiefly dependent upon the focal length of the 

 objectives not upon the power of the eye-pieces. The employ- 

 ment of deep eye-pieces i.e., such as magnify 10 linear or 

 upwards is, as a rule, advantageous only in cases where the 

 determination of form-relations or the comparison of very small 

 distances is required. New details in the image will be less per- 

 ceptible the more imperfect the objective and the greater the length 

 of the body-tube; in most cases the highest-power objectives 

 exhibit decidedly less of the image than the preceding lower ones. 



The most suitable amplifications for particular kinds of work 

 will soon be discovered by those who use the Microscope for 

 scientific purposes. It is manifest that immersion systems and 

 similar high powers will be employed only in special cases, where 

 very difficult peculiarities of structure, &c., have to be determined. 



3. EMPLOYMENT OF COVER-GLASSES. 



Most microscopic preparations are examined in water or other 

 fluid medium, either to lessen the deviation of the rays of light, 

 or to keep the object in a settled condition soft, distended, &c. 

 1 Vide Strasburger : " Ueber Zellbildung und Zelltheilung," p. 33. 



