428 



OPTICAL INSTRUMENTS. 



more than its focal distance ( 630) beyond the slide. A 

 real, inverted, magnified image of the picture is thus pro- 

 jected upon the screen 8. The tube carrying m is adjust- 

 able, so that the foci may be made to fall upon the screen 

 and thus render the image distinct. By inverting the 

 slide the image is seen right side up. The solar and 



electric microscopes 

 act in nearly the same 

 way, the chief differ- 

 ence being in the 

 source of light. 



(a.) Directions for 

 making a simple magic 

 lantern may be found on 

 page 84 of Mayer and 

 Barnard's little book on 

 FIG. 334. Light. Fig. 334 repre- 



sents a very compact and 



efficient lantern, known as Marcy's Sciopticon, and furnished by 

 Ritchie, of Boston. (See Dolbear's Art of Projecting.) 



665. Stereoscopic Pictures. Close the left eye 

 and hold the right hand so that the forefinger shall hide 

 the other three fingers. Without changing the position 

 of the hand, open the left and close the right eye. The 

 hidden fingers become visible in 

 part. Place a die on the table 

 directly in front of you. Look- 

 ing at it with only the left eye, 

 three faces are visible, as shown 



FIG. 335. 



at A, Fig. 335. Looking at it with only the right eye, it 

 appears as shown at B. From this we see that when we 

 look at a solid, the images upon the retinas of the 

 two eyes are different. If in any way we combine two 



