164 SCIOPTICON MANUAL. 



perly matched and in line, will be just before the margin 

 would become tinged with yellow. 



When ready to project the views, intensify the light, if 

 necessary, by letting on more hydrogen followed by more 

 oxygen, till exactly the best effect is produced. During the 

 exhibition, when the lime begins to fail at the point exposed, 

 lifting up the back of the wheel about half an inch will 

 bring down a fresh but hot surface into place, which may 

 be done to advantage once in five or ten minutes, more or 

 less, according to the quality of the lime and the force of 

 the jet. The wheel lies loosely in its holder, and being a 

 non-conductor, is not very hot at the back, so it can be con- 

 veniently turned by the thumb and finger. 



RATIO OF SPACES. 



In the ordinary Sciopticon arrangement, the spaces in 

 inches are about 3 from the focus of illumination to the 

 first face of the condenser, 2 through the condenser, 5 be- 

 tween the condenser and objective, and 3 through the ob- 

 jective tube. The focus of the condenser is 3, or the back 

 focus, 2 ; that of the objective is 51, or from the back glass, 

 41-. The conjugate foci of the condenser, measuring from 

 the light to about 3 inches front of the objective, are 4 and 

 12. The conjugate foci of the objective, measuring from 

 the picture slide to the screen, 20 feet away, are about 6 

 and 240. Multiplying the diameter of the slide picture 3 

 by the ratio ^, gives 120, or 10 feet; so, in general, the 

 diameter of the disk on the screen is about half the measure 

 of its distance. For example: if the screen is 9 feet wide, 

 the instrument may be 18 feet away. If the slide pictures 

 vary in size, the images, of course, will vary in proportion. 



With the long focus arrangement, for long distances, a 

 condenser is used of 2* inches back focus, and an objective 

 of 10 inches back focus. With these lenses the diameter 



