CH. IX] HIGH POWER MICRO-PROJECTION 283 



be too dim if the screen distance is over two or three meters (6 to 10 

 feet). 



With objectives of 4, 6, 8, 10 mm. and lower powers, one can 

 use a greater distance with satisfaction, but for the oil and water 

 immersions, a distance of one to two meters (3 to 7 feet) gives the 

 best results. This, of course, refers to minute details. If one 

 simply wants size, the limit is much greater; but that is not 

 scientific projection. 



409. Kind of screen for high power projection. The prin- 

 ciple enunciated by Goring and Pritchard must be kept in mind. 

 The whiter and smoother the screen, the more brilliant the image 

 and the clearer the details. Nothing has been found better by 

 the writers than smooth, white bristolboard. This is also very 

 easily procured, and when it becomes dirty or discolored, it can be 

 cheaply replaced. We have also found white cardboard in sheets 

 of 71 x 112 cm. (28 x 44 in.) good. 



410. Specimens to project with high powers. These must 

 have in a good degree the qualities of specimens giving clear images 

 to the eye in direct, microscopic observation. That is, they should 

 have definite outlines and contrasting colors; for example, well 

 stained preparations of red and white blood corpuscles mounted in 

 balsam and projected with the oil immersion objective. 



Preparations of bacteria, well stained and mounted in balsam, 

 may be projected with the oil immersion. 



Thin histologic and embryologic sections, if well stained and 

 mounted in balsam, answer well. The nuclei of cells show well, 

 also the band of cilia in a ciliated epithelium, and the cells in 

 mitotic division. Naturally, well prepared plant preparations 

 have the advantage of very sharp outlines. 



411. High powers with the vertical microscope. Any prep- 

 aration which can be projected well with high powers may be used 

 on the vertical microscope ( 397). Of course, there is some loss 

 of light in the double reflection required (fig. 147, 176), but if the 

 screen is within two meters (6 ft.) distance and the observers few 

 and close, results are fairly satisfactory. For example, if one has 



