174 AVOIDANCE OF INVERSION; ERECT IMAGES [Ch. VI 



turn the drawing paper around 180 and the letters or figures will 

 appear erect. 



Put the metric card wrong edge up to start with; then the letters 

 or figures will appear right side up on the tracing paper. 



§ 279. The use of a negative for projection and tracing. — Put 

 the face of the negative that reads correctly next the source of light 

 and wrong edge up : then it will appear erect in every way on the 

 drawing paper. This is the way lantern slides are put in the 

 holder. 



§ 280. The Wollaston or Abbe camera lucida. — With these 

 camera lucidas there are two reflections of the rays (fig. 99-100), 

 consequently there is no inversion produced by the camera, but the 

 microscope inverts the image the same as the photographic objective, 

 and erect images are obtained either by inverting the drawing after 

 it is made or by putting the object in an inverted position under 

 the microscope, just as with the photographic camera. 



Demonstrate that this will produce erect drawings by using the 

 letters (fig. 105) and making sketches of their images by the camera 

 lucida, having the letters right edge up on the stage in one case and 

 wrong edge up in one. 



Erect Images with the Projection Microscope 

 § 281. Erect images with an objective only or with an objective 

 and amplifier. — There are two cases: (1) When opaque drawing 

 paper is used. In this case the object must be put on the stage 

 with the cover-glass toward the light and the slide toward the ob- 

 jective, and it must be wrong edge up. Only low powers (16 mm. 

 and lower objectives) should be used, for the thick slide introduces 

 aberrations (fig. 51) and is liable to be too thick for the free working 

 distance (fig. 31). 



(2) When a translucent drawing paper is used and the drawing 

 is made on the back. In this case the specimen is put on the stage 

 wrong edge up, but with the cover-glass facing the objective. All 

 powers can be used. This is similar to the conditions described for 

 the photographic camera where the tracing paper is used on the clear 

 glass (§ 278). 



