226 



MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX 



objective with larger lenses admits much more light, hence the 

 screen image will be brighter. For example, suppose it were 

 desired to obtain the magnification which is given by a 2 mm. objec- 

 tive, it would be much better to use a 4 mm. objective and an 

 amplifier doubling the size of the real image. This would make the 

 screen image of the same magnification as the 2 mm. would give, 

 and it would be far brighter and show a larger field. In like 

 manner and for the same reason, it is better to use an 8 mm. objec- 

 tive and an amplifier, than a 4 mm. objective without the amplifier 

 (but see 401). 



A B C D 



FIG. 124. FIGURES SHOWING THE GENERAL CONSTRUCTION OF MICROSCOPE 



OBJECTIVES. 



A Low power objective of a single combination (50-30 mm. equivalent 

 focus). 



B, C Medium power objectives with two combinations (25-12 mm. 

 equivalent focus). Sometimes the front combination is composed of two and 

 sometimes of three lenses as shown. 



D High power objective (8 to 2 mm. equivalent focus). Usually the front 

 combination is of a single lens, the others of two or three lenses as shown. 

 Many high power objectives have but three combinations. 

 (D is from Voigtlander's Catalogue). 



The writers have found that in projection for actual class demon- 

 strations, objectives of higher power than 4 mm. arc unsatisfactory. 

 We believe also that the purpose of class-room projection is not 

 the demonstration and study of minute details which require that 

 the observer should be close to the screen image, but the general 

 outlines and broad features which can be seen clearly at a distance 

 when suitably magnified. 



