CH. III.] INTERPRETATION OF APPEARANCES. 83 



small piece of glass about as thick as a slide. The second letter is 

 placed upon this and covered in like manner. The third letter is placed 

 upon the second thick cover and covered with an ordinary cover-glass. 

 The letters should be as near together as possible, but not over-lapping. 

 Employ the same ocular and objective as above (§ 121). 



IvOvver the tube till the objective almost touches the top letter, then 

 look into the microscope, and slowly focus up. The lowest letter will 

 first appear, and then, as it disappears, the middle one will appear, and 

 so on. Focus down, and the top letter will first appear, then the mid- 

 dle one, etc. The relative position of objects is determined exactly in 

 this way in practical work. 



For example, if one has a micrometer ruled on a cover-glass 15-25 

 hundredths 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 

 microscope and uses a 3 mm. (^th in.) objective, it is easily deter- 

 mined. The cover should be laid on a slide and focused till the lines 

 are sharp. Now, without changing the focus in the least, turn the 

 cover over. If it is necessary to focus up to get the lines of the microm- 

 eter sharp, the lines are on the upper side. If one must focus down, 

 the lines are on the under surface. With a thin cover and delicate 

 lines this method of determining the position of the rulings is of con- 

 siderable practical importance. 



§ 125. 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 he could not tell whether it was a 

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

 he 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 solids. So for micro- 

 scopic 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 histology, and the microscopist who neglects 

 to see all sides of the tissue elements has a very inadequate and often a 

 very erroneous conception of their true form. 



§ 126. Transparent Objects having Curved Outlines. — The sue- 



