122 EXPERIMENTS FOR INTERPRETATION [Ch. IV 



ance under the microscope of objects having geometrical patterns 

 and especially if there are several of them superimposed, as with the 

 wire gauze experiment (§ 216). The image is often just as satis- 

 factory in one focus as in another, although the appearance changes 

 very markedly in the two positions. 



§ 213. Comparing two microscopic fields side by side. — It is so 

 difficult to carry in the mind the exact appearance of any structure 

 or complex pattern, that many efforts have been made to have the 



microscopic images side by side so that 

 they can be looked at at the same time. 

 This has been accomplished by using 

 two microscopes and projecting two fields 

 side by side, as can be done by having 

 two microscopes like the one shown in 



fig- "3- 



Another method is by means of a com- 

 parison ocular (fig. 74). Then two objects 



Fig. 73. Geometrical un der two microscopes have the images 

 Pattern to Show Dif- ... . , , , , ,, , ,- , , 



ference of Appearance side by side in the ocular, fiali tfie field 



Depending on the Focus. De i ng taken up by one object and half by 



(From Sir A. R. Wright's the other; then the eye can compare two 

 Microscopy). 



structures side by side. 



§ 214. Muscae volitantes. — These specks or filaments in the 

 eyes due to minute shreds or opacities of the vitreous humor some- 

 times appear as part of the object as they are projected into the field 

 of vision. They may be seen by looking into the well-lighted micro- 

 scope when there is no object under the microscope. They may 

 also be seen by looking at brightly illuminated snow or other white 

 surface. By studying them carefully it will be seen that they are 

 somewhat movable and float across the field of vision, and thus do 

 not remain in one position as do the objects under observation. Fur- 

 thermore, one may, by taking a little pains, familiarize himself with 

 the special forms in his own eyes so that the more conspicuous at least 

 may be instantly recognized. 



§ 215. Miscellaneous observations. — In addition to the above 

 experiments it is very strongly recommended that the student follow 



