and Laboratory Methods. 



2143 



be more appreciated when one has an opportunity to run through a long series 

 of papers on different subjects, such as are found on the programs of society 

 meetings or in the lecture room in a large educational institution during a day's 

 work. The projection microscope used with this apparatus is of very simple 

 construction, being simply a microscope fine adjustment and coarse adjustment 

 with body tube and stage, as in an ordinary microscope, but instead of the pillar 

 and base a special arm which adapts it to one of the supporting blocks identical 

 with those which carry the other optical parts. The stage is provided with a 

 sub-stage actuated by quick acting screw, in which condensing lenses of various 

 foci can be used for different power objectives. The microscope lenses employed 

 for projection work with this apparatus consist of microscope objectives having 

 a somewhat less numerical aperture than those usually employed for visual pur- 



FiG. 6. — End view of combined slide and microscope object projector. 



poses, thus securing greater depth of focus or penetration, increasing their value 

 for thick objects, and especially corrected to eliminate spherical aberration and. 

 color* without special regard to chemical focus, as this is of no consequence in a 

 projection lens. The mounts are perfectly straight and provided with a sliding 

 hood carrying a diaphragm. The size of the aperture in this diaphragm is 

 regulated to cut off the edge of the field, so that a perfectly sharp and 

 well defined circle appears on the screen, as (without this diaphragm) the 

 image would gradually fade off, giving an unpleasant effect. When, however, it 

 is desired to increase the size of field for regional demonstrations, etc., it is only 

 necessary to slip the hood off from the objective and replace it again when 

 needed. Powers of from three inches down to one-half inch are used with great 

 facility on ordinary screens, at the ordinary distance at which ordinary projec- 

 tion lenses are used, giving ample illumination for observation, even in the rear 



