676 SUMMARY OF CURRENT RESEARCHES RELATING TO 



If in course of time the electric light is more perfected a new 

 epoch will commence for the lantern Microscope, and this highly 

 interesting instrument, in a compendious form, will certainly not be 

 wanting in any wealthy (gebildeten) family. 



The objectives must of course be as free as possible from spherical 

 and chromatic aberration, and must form a perfect image of the 

 object, not only in and near to the axis (as in the ordinary compound 

 Microscope), but over the whole extent of the image-surface, a by no 

 means easy matter with large apertures. 



Dr. Schroder has constructed a projection-Microscope^ for the 

 Microscopical Aquarium at Berlin, and a considerably more improved 

 one for North America, the first of which is shown in Fig. 120. 



Fig. 120. 



The source of light is at a ; b and c are plano-convex lenses of 

 crown glass, between which at p an alum cell is interposed to inter- 

 cept the heat rays. The rays emerge from c strongly convergent, but 

 are made parallel and corrected for spherical and chromatic aberra- 

 tion by the combination d e. The parallel beam s is made convergent 

 by the movable lens / according to the requirements of the field of 

 view. For polarized light a large Nicol prism can be placed at s, 

 and selenite plates at u. The analyser is attached to the objective. 



By means of a silver prism g the illuminating beam is thrown 

 upon the object I vertically " in order to admit of using receptacles 

 for holding living animals in fluid, &c." 



The objectives m, ml are attached to a revolving holder o, o'. 

 Powers from 100 to 2000 can be used. They are focussed by the 

 screw n, the upright piece t serving for revolving the holder when a 

 different power is required. 



The rays after having passed through the objective are reflected 

 by a silvered prism h horizontally through the negative achromatic 

 lens ti, and form an image at q. 



The American instrument has immersion lenses giving a power of 

 4000 times, and can be used for opaque objects by means of a large 

 Lieberkuhn. 



"Notwithstanding the many reflecting surfaces," Dr. Schroder 

 says that " with only an ordinary petroleum lamp the larger diatoms 

 such as Triceratium favus can be very distinctly seen. With the oxy- 



