and Laboratory Methods. 2145 



of a good sized room, and with circles up to eight or nine feet in diameter. 

 When, however, it is desired to use higher powers, the best results are obtained 

 by using smaller circles, perfectly opaque and perfectly flat, white screens, or 

 better still, semi-transparent screens for the very highest powers, and placing the 

 lantern facing the audience, projecting the image so that it will be viewed 

 through the screen. In this way it has been possible to show bacteria with a 

 J2-inch oil immersion lens to audiences of considerable size. 



The apparatus, as thus far described, is intended for projection of such 

 objects as can be stood upon edge and projected in the ordinary manner, 

 preferably permanent microscopical preparations and ordinary lantern slides. 

 Where, however, it is desired to show living forms under the microscope, or 

 under the ordinary projection lens in fluid, etc., an apparatus for diverting 

 the beam from the condensing lenses to a vertical path, and then bringing 

 it again to the horizontal axis, is required. For this purpose a horizontal 

 attachment to the projection apparatus has been devised, a diagram of which 

 is shown in Fig. 7. This attachment is intended for use with either the 

 projection lens or projection microscope. It consists of a large plane mirror 

 with a very perfect surface set at an angle of 45°, mounted in a light tight box 

 and carried on a rectangular arm. This is attached to the base bar of the pro- 

 jection apparatus by a T piece, as the other attachments are. The vertical arm 

 for this attachment is arranged to hold the microscope and projection lens used 

 for regular projection, and, when placed upon the arm, the microscope and 

 projection lens come into the optical axis of the beam of light projected upward 

 from the mirror. For use with the projection lens, the front piano convex 

 condensing lens is lifted out of the condenser system, it being mounted in a ring 

 with knob for this purpose, and placed with its plane surface uppermost in the 

 cell above the inclined mirror. A plane plate of glass mounted in a suitable ring 

 is then placed over the condensing lens to protect it, and the objects to be 

 projected are placed upon this glass plate. The projection lens is focussed 

 upon it, and upon the hood of the projection lens a 45° prism is placed 

 which diverts the vertical beam to a horizontal path, throwing the magnified 

 image upon the screen. A small percentage of light is lost in these reflec- 

 tions, but where the apparatus is perfectly centered, and the prism and mirror 

 surfaces are very accurately made, the percentage is so small that with the 

 arc light no difiticulty is experienced in projecting lantern slides and all 

 ordinary translucent objects at ordinary distances, and securing circles of 

 the same size as with the ordinary projection lens. At about twenty feet 

 distance, with the one-fourth projection lens, magnifications of about 80 

 diameters are had, and an object over four inches in diameter can be 

 shown, so that one can take a four-inch Petri dish for example, filled with 

 water, and place in it a series of frog larva; from the egg up to forms with 

 legs and demonstrate the whole series at one operation. The formation of 

 crystals, the observation of dissections and hundreds of other uses will sug- 

 gest themselves for an apparatus of this kind. When the projection micro- 

 scope is swung into position, the piano convex condensing lens is returned to 

 the front of the condenser system, and its protecting plate is replaced by a 



