Ch. IV] DETERMINATION OF THE CHARACTER OF OBJECTS ill 



For example, if one has a micrometer ruled on a cover-glass 0.15- 

 0.25 mm. thick, it is not easy to determine with the naked eye which 

 is the ruled surface. But if one puts the micrometer under a micro- 

 scope and uses a 4 mm. objective, it is easily determined. The cover 

 should be laid on a slide and focused till the Unes are sharp. Now, 

 without changing the focus in the least, turn the cover over. If it is 

 necessary to focus up to get the hnes of the micrometer sharp, the lines 

 are on the upper side. If one must focus down, the lines are on the un- 

 der surface. With a thin cover and deUcate Unes this method of deter- 

 mining the position of the rulings is of considerable practical importance. 



§ 192. Determination of the form of objects. — The procedure is 

 exactly as for the determination of the form of large objects. That 

 is, one must examine the various aspects. For example, if one were 

 placed in front of a wall of some kind, one could not teU whether it 

 was a simple wall or whether it was one side of a building unless in 

 some way one could see more than the face of the wall. In other words, 

 in order to get a correct notion of any body, one must examine more 

 than one dimension, — two for plane surfaces, three for soUds. So 

 for microscopic objects, one must in some way examine more than 

 one face. To do this with small bodies in a liquid the bodies may be 

 made to roll over by pressing on one edge of the cover-glass. And in 

 rolling over the various aspects are presented to the observer. With 

 solid bodies, like the various organs, correct notions of the form of 

 the elements can be determined by studying sections cut at right 

 angles to each other. The methods of getting the elements to roll 

 over, and of sectioning in different planes, are in constant use in His- 

 tology, and the microscopist who neglects to see aU sides of the tissue 

 elements has a very inadequate and often a very erroneous" conception 

 of their true form. 



§ 193. Transparent objects having curved outlines. — The success 

 of these experiments will depend entirely upon the care and skiU used 

 in preparing the objects, in hghting, and in focusing. 



Employ a 4 mm. or higher objective and an 8x or lox ocular for 

 all the experiments. It may be necessary to shade the object (§ 140) 

 to get satisfactory results. When a diaphragm is used the opening 

 should be small and it should be close to the object. 



