CH. IX a} PROJECTION MICROSCOPE 261 



50, 42, 35, and 25 mm. focus very excellent. Those found most satis- 

 factory by the writer are the so called projection objectives to be used 

 without an ocular. For higher powers than 25 mm. the ordinary 

 objectives used on the microscope are mostly employed and, when 

 properly selected, are very satisfactory. 



For class demonstration, as stated above, one can show to an 

 audience of 200 with a screen distance of 5 to 8 meters all the details 

 which show satisfactorily with a 16 mm. objective and low ocular 

 under the ordinary microscope. One rarely uses with real satisfaction 

 an objective higher than 3 or 4 mm. equivalent focus. One of from 

 6 to 10 mm. focus is mostly used. For fine structural details one 

 must use an ordinary microscope. (Bee the table, 425 for size of 

 field and magnification). 



For powers above 3 mm. it is more satisfactory to have a screen 

 distance of 3 to 5 meters or even less. Such demonstrations are only 

 for small numbers, and they must be near the screen or use opera 

 glasses. 



414. The Use of Oculars in Micro-Projection. The writer 

 has found projection without oculars more satisfactory then by their 

 use. The field is larger and higher objectives can be used, but the 

 oculars have the advantage of giving increased magnification so that 

 one can get the same amplification with lower objectives. A further 

 advantage is that the outlines of the field will be sharp and the part of 

 the specimen shown will be practically all in focus. Besides restrict- 

 ing the field, however, the oculars reduce the light greatly. Without 

 the ocular the more brilliant image, and the larger field are very 

 advantageous. With the larger field the whole of many objects may 

 be seen and the relations of the special part under examination noted 

 at a glance. It may be repeated also that the micro-planars and 

 the objectives of low power made especially for projection and photo- 

 graphy, are designed for use without an ocular. In using these 

 objectives their iris diaphragm must be opened as fully as possible. 



415. Kind of Oculars. If an ocular is used it may be either 

 the projection oculars like those for photography (see 372, A, B), in 

 which case the diaphragm of the ocular must be focused on the screen 

 to get the best results ( 372, B), or one may use a compensation or an 

 ordinary huygenian ocular. From the writer's experience an objective 

 above 8 mm. focus cannot be used satisfactorily for demonstration with 

 an ocular. If oculars are used in projection it is better to have a screen 



