2142 Journal of Applied Microscopy 



out decentering the arc, by this means, and it is a very quick and valuable 

 method of adjusting the illumination. The placing of the carbons at an angle of 

 90° to each other with the horizontal carbon in the optical axis, not only throws a 

 greater volume of light from the crater of the positive carbon through the con- 

 densing lenses, but retains the glowing crater always exactly in the optical axis. 

 No matter how irregularly the two carbons may burn, so long as the arc passes 

 between them, the point of greatest intensity for light radiation will be in the 

 optical axis. Any desired current can be used with a hand-feed lamp of this 

 character, as there are no coils or magnets connected with it. 



An automatic electric arc light is also in process of construction in which the 

 carbons are placed at an angle of 90° to each other, the horizontal carbon being 

 placed in the optical axis. Experimental tests with this lamp so far conducted 

 show that it will burn without attention for at least three hours, and that, its 

 period of regulation being very short, the variations in potential in the arc are 

 also very slight, hence the intensity of the arc remains practically unaltered, and 

 the flickering, hissing and other irregularities observed in lamps in which a con- 

 siderable length of time elapses between the movements of the carbons seem to 

 be almost entirely absent. This lamp is arranged to adjust vertically, laterally 

 and horizontally, just as the hand-feed lamp does, and while it is constructed to 

 operate on 110 volt direct current and gives good results with 10 or 12 amperes, 

 the automatic feed may be entirely cut out and the lamp operated as a hand- 

 feed lamp with independent movement of each carbon, on either direct or indi- 

 rect current, and with any desired voltage. 



The crucial test of the whole apparatus, however, lies in the substitution of 

 the projection microscope, as in Fig. 5, for the ordinary projection lens. In this 

 case, where microscope objectives are used, in which the front lenses are from 

 1 to 2 or 3 millimeters only in diameter, the whole optical apparatus must be 

 absolutely centered and the radiant placed exactly in the optical axis in order to 

 obtain good results. The construction proves itself accurate to such an extent 

 that it has been possible to build upon one of the supporting bases a double pro- 

 jection apparatus, consisting of two arms at right angles attached to a common 

 center, one bearing the ordinary projection lens for projecting lantern slides, and 

 the other a projection microscope, the construction being shown in Fig. 6. It 

 will thus be seen that either the projection microscope or projection lens can be 

 swung into the optical axis and the accuracy of the centering and stability of the 

 whole is such that when the projection lens is focussed upon an object in the slide 

 carrier, and the projection microscope is focussed upon an object on the microscope 

 stage, the light being suitably arranged for either, the other may be swung into 

 position, and its image instantly projected upon the screen ; thus, if the demon- 

 strator is desirous of showing, for example, an illustration of hydra, representing 

 it in its living form., with its surroundings, he can do so by means of a lantern 

 slide and instantly substitute upon the screen for this picture the image 

 of a cross section of the hydra itself by means of the projection microscope. 

 Any part of a lecture can be illustrated with lantern slides and microscopic sec- 

 tions can be interspersed between them. The value of this attachment permiting 

 the use of the projection microscope or the ordinary projection lens at will, will 



