?io OPTICAL PROJECTION 



such as described in 96, used thus, will give considerable 

 power, with more ' depth of focus,' and less trouble in 

 ' finding.' The best general distance will be 12 to 15 feet, 

 beyond which the performance of every high-power lens 

 rapidly deteriorates. Even brilliance depends much upon 

 crispness, and therefore a Eeichert's TA (j) at 12 feet will 

 give both more light and better work (though the image is same 

 size) than his No. 6 (-}) at 17 feet. When the highest power 

 is needed, it is usually better to use a good oil-immersion lens 

 than to increase the distance. This refers to the magnification 

 of minute detail; on the contrary, if the object be to exhibit 

 ordinary objects on a colossal scale, this is generally better 

 done by a screen distance of not less than 25 feet, with 

 amplifiers or high-power eye-pieces, or often by stretching 

 the distance to 50 feet or more. In this way a flea may be 

 shown 30 or 40 feet long with ease. But for histology, which 

 will always be demonstrated to moderate audiences, short 

 distances and high powers are far the best. Zeiss's projection 

 oculars will give good results with most wide-angled lenses, 

 where their very contracted field is sufficient for the purpose ; 

 but it will save time to centre the slide with one having a 

 larger field. 



Ample light is important, and I would prefer 3,000 candle- 

 power to less. For a material point is to stop down the 

 cone of light from the substage condenser, with the iris 

 diaphragm, to the precise degree which gives the best result. 

 Mr. Nelson's experiments have determined that even on the 

 compound microscope this is only about three-fourths of the 

 angle of the objective ; in projection it will rarely exceed two 

 thirds. A cone will always be found that gives a distinctly 

 best result, focussed on the object. That is the chief point- 

 about the microscopic manipulation. 



With powers approaching 4,000 or 5,000 diameters, all 

 that has been said in 101 applies with increased force. 

 Histological detail will depend, even more than with the 



