2012 Journal of Applied Microscopy 



The Technique of Biological Projection and Anesthesia of 



Animals. 



Copyrighted. 



VII. PROJECTION MICROSCOPES USING ELECTRIC ARC OR OXYHYDRO- 



GEN LIGHT— Continued. 



Higher power objectives are to be tested in the same manner after the sub- 

 stage condenser has been put into place. In working with high power objec- 

 tives, it is necessary to keep in mind that the fields of such lenses are small, 

 being only J^ of an inch in diameter in a ^i^^-inch oil-immersion, and that this 

 small field must be perfectly illuminated for successful work. It is readily seen 

 by observing an arc for a time through smoked glass, or a combination of two 

 plates of deep ruby and blue glass, that the arc creeps about on the ends of the 

 carbons as they are slowly consumed. This variation in the position of the 

 luminous point carries it away from the optical axis at the lamp and shifts it a 

 corresponding distance on the opposite side of the axis at the object. There 

 are three ways for overcoming this difficulty. First, by using soft-cored carbons ; 

 second, by using as small a horizontal carbon as possible in combination with a 

 larger vertical carbon ; and third, by moving the luminous point into the optical 

 axis by a slight vertical or lateral movement of the arc. To accomplish the last, 

 the vertical supporting rod of the lamp should not be clamped rigidly and a slight 

 lateral push on the feed-wheels will rotate the lamp on its support. Small carbons 

 burn away more rapidly than large ones and the feed requires more frequent 

 attention. The carbons shown in the engraving are ,",y and ^\. inches in diam- 

 eter, " Nurnberg soft-cored, electra brand." In the highest power work which the 

 writer has done with the electric light, a very steady light was obtained by using 

 a ^-inch cored vertical and a y'^-inch solid horizontal carbon. The light from 

 alio volt alternating incandescent current was sufiici«nt to give a strong picture 

 of a stained transverse section of an earthworm, having a magnification of 8800 

 diameters by measurement. The objective used was a B. & L. J^-inch oil-im- 

 mersion in connection with an amplifier. 



The conditions attending the production of the best light, as described 

 above, indicate the reason for the use of hand-feed 90° arc lamp rather than 

 any of the automatic-feed lamps or other types of hand-feed lamps. With the 

 90° hand-feed lamp the arc is under the control of the operator at every instant, 

 while with the automatic-feed the operator is frequently under the control of the 

 feeding mechanism at the very instant when his live specimen is at its best. 

 The automatic-feed lamp is convenient in very low power and lantern slide pro- 

 jection, but here also the 90° lamp gives as good results. 



Concerning the field of illumination on the screen it should be noted that, 

 if the microscope is pushed too close to the lamp, the field will be blue and will 

 not give satisfactory results. As the microscope is moved to a greater distance 

 from the lamp, the center of the field will be strongly illuminated, and at a still 

 greater distance the entire field will be evenly illuminated, and this is the best 



