CH. X] ERECT IMAGES IN DRAWINGS 361 



lenses is used. The earliest workers also saw that an easy way to 

 correct for this was to invert the object, then its image would 

 appear in the natural position. But some objects do not admit of 

 inversion, hence the effort to obtain erect images by some optical 

 means. 



The first and still the simplest method is by the use of a plane 

 mirror with a horizontal screen (fig. 88, 89, 181, 204). The mirror 

 might be put in the course of the beam before or after it has passed 

 the objective. Figure 89 shows it before and figure 182 after 

 traversing the objective. 



It was demonstrated by Kepler (1611) and practically worked 

 out by Scheiner (1619) that erect images could be produced by the 

 use of two objectives in line. The first objective gives a real 

 inverted image of the object, and the second gives a real, erect 

 image of the inverted image (fig. 208). This is what occurs when- 

 ever an ocular is used with an objective in projecting with the 

 microscope (fig. 207). 



The principles for getting erect images with projection apparatus 

 are very simple, but in practice it is a little puzzling to decide off- 

 hand just how to arrange the object so that the screen image shall 

 be erect and not show any of the inversions (fig. 212-214). This 

 difficulty arises from the fact that in the different kinds of projec- 

 tion sometimes an opaque object is used, and sometimes a trans- 

 parent object ; sometimes an opaque and sometimes a translucent 

 screen is employed; sometimes an objective only, and sometimes 

 both an objective and an ocular are used for projecting the image; 

 and finally, sometimes it is necessary to use a mirror or prism as well 

 as an objective to get the image on the vertical or horizontal surface 

 where it is to be seen or drawn. 



The simplest and surest way to get the microscopic specimen on 

 the stage of the projection microscope in a position which will give 

 a correct image for drawing is the following: 



i. The prepared microscopic specimen is placed on a piece of 

 white paper so that it appears exactly as it should in the drawing, 

 and the letters a and k are written on the cover-glass between 

 the specimens (fig. 220). 



