5Q8 IMAGE FORMATION, PROJECTION MICROSCOPE [Cn. XIV 



purpose and was not intended to serve for lantern slides also, the 

 design would be exactly similar to that for lantern-slide projection 

 except that everything would be on a smaller scale, the condenser 

 lenses being of smaller diameter and of shorter focal length. This 

 would, of course, necessitate placing the lamp very close to the 

 lenses, but they will be small and correspondingly thin and will not 

 crack as easily as larger ones. Whether or not this would be a good 

 design for a large size outfit using 35 to 50 amperes is not certain, 

 but there is no doubt that good results can be obtained for projec- 

 tion on a small scale using three to four amperes which would not 

 be possible on account of the difficulty of getting even illumination 

 if the big standard size condenser were used. 



832. Experiment with small size condenser. The method 

 of image formation using a small size condenser is shown in fig. 336. 

 L,is the source, an arc using 5 mm. carbons and 3 amperes of cur- 

 rent. It is practically a point source. Condenser lenses 58 mm. 

 (2>m.) in diameter and 63 mm. a^in.) focal length placed 25 mm. 

 (i in.) from the source were used. 



Even when using this very small source (3 mm. circle) a perfectly 

 uniformly illuminated field was obtained, a thing which could not 

 be done when a large condenser having the usual amount of 

 spherical aberration was tried. 



The diameter of the light cone through the objective was 2 cm. 

 (24 in.). When large carbons were used and the current increased 

 to 1 2 amperes the effects were to increase the brightness of the pic- 

 ture and to increase the diameter of the cone of light through the 

 objective to 3 cm. (i^ in.). It is seen that in either case the lenses 

 of the objective did not need to be of as large diameter as when 

 using the ordinary large condenser. 



IMAGE FORMATION WITH THE PROJECTION MICROSCOPE 



833. Illumination for low powers. For low powers (20 to 100 

 mm. objectives) the principle is that the focus of the condenser 

 should fall at the center of the projection objective and that the 

 object should be placed in the converging cone of light in the posi- 



