366 



ERECT IMAGES IN DRAWINGS 



[Cn. X 



kind or position of a screen is to use a specimen prepared as follows : 

 An ordinary microscopic slide is varnished as directed for lantern- 

 slide glasses (Ch. VIII, 317) and then the small letters a and k are 

 written in the middle with a fine pen. These letters are selected 

 because both in script and in printing they indicate clearly which 

 side up they are and which way they face. With some letters it is 

 not so easy to determine whether they have suffered an inversion 

 or not. 



A low power, 50 to 100 mm. focus objective, is good for projecting 

 the image. 



One could use a lantern slide with print upon it, or even a nega- 

 tive. For our experiments we used a lantern slide or negative of 

 the metric measure (fig. 178, 211) so that cuts could be made for 

 this book which were exactly like the images obtained on the screen 

 with the transparency in different positions. 



Ocular 



Objective 



FIG. 207. DIAGRAM OF THE COURSE OF THE RAYS AND THE POSITION OF 

 THE IMAGES WHEN AN OCULAR is USED. 



Object The object whose image is to be projected. 



Objective The projection objective. 



// Field lens of the ocular. It acts with the objective to give a real, 

 inverted image r i. 



r i The real, inverted image of the object formed by the objective and the 

 field lens of the ocular. 



r 1 *'* The inverted image of the object which would be formed by the objec- 

 tive if the ocular were absent. 



e I Eye lens of the ocular. It acts like a second projection objective and 

 gives a screen image of the real image (r i). 



Axis The optic axis of all the lenses. 



Screen Image The image projected by the eye lens. This image will be 

 right side up, but the rights and lefts will be reversed on the ordinary opaque 

 screen. If a translucent screen is used and the observer is behind it, the image 

 will appear erect, and the rights and lefts will not be reversed. 



