254 L"SE OF PROJECTION MICROSCOPE [Cn. IX 



the cover-glass and not on the slide for the reason that if it were put 

 on the slide it would almost entirely overcome the good effect of the 

 stage cooling cell, as it would hold the slide away from the glass 

 surface, so that the heat could not be carried off by conduction. 

 If it is on the cover-glass, the slide can then rest directly against 

 the stage water-cell. 



If one ever wishes to remove the mask it is easily done by putting 

 a piece of wet blotting paper upon it till thoroughly softened. It 

 can then be peeled off, and the cover-glass cleaned with a wet cloth. 



387. Field of view in the screen image. Except with objec- 

 tives corrected in the manner of photographic objectives the screen 

 image will not be equally sharp over the entire field where the large 

 tube and where no tube is used (fig. 138, 145). To obviate this, 

 oculars may be used, or iris diaphragms to cut off the outer margin 

 which is not sharp. This margin also shows color from the 

 chromatic aberration of the condenser. But demonstrations in 

 histology and embryology, at least, depend largely in their effec- 

 tiveness upon the relations of parts shown in a large field. The 

 part to be shown with greatest distinctness is brought into the 

 middle of the field as with ordinary microscopic observation. 



The importance of a large field in which the relation of parts can 

 be shown, can be illustrated by a simple experiment. For example, 

 let a well known friend cover his face with a mask having only eye- 

 holes, or with a hole to show a part of the cheek or forehead. It 

 would be hard to recognize him from that limited view alone. 



388. Objectives needed for different sizes of field. In fig. 

 1 44 there is given a graphic representation of different sizes of field 

 or object which one might wish to project, and the objective or 

 objectives with which it can be done. It will be seen that the 

 larger the field the longer must be the focus of the projection 

 objective. In this figure it is assumed that no ocular is used and 

 that the field is not restricted by the tube of the microscope, hence 

 for the largest fields the objective must be mounted in a shield 

 without tube (fig. 138). In fig. 137 is shown how the field may be 

 cut down by using microscope tubes of different diameter. See 

 also the table of magnification and field ( 391). 



