THE OBJECT 101 



aperture gives sharp and brilliant pictures, even when it is 

 also nearly 1,000 times the objective aperture. With less 

 careful microscopy, a limit of 500 times the objective 

 aperture is sometimes not to be attained together with 

 satisfactory pictures. 



Magnifications above the upper limit, unless the objective 

 and adjustments are perfect, may only soften the image. 

 In any case, they show nothing new to normally sharp 

 sight, though they are useful for tired or dimmed vision. 

 Some well-adjusted objectives of 1.3 aperture will, in 

 the writer's experience, give enlargements sharply up to 

 a maximum of about 1,600 or even 1,800, diameters, 

 with a perfect condenser cone. It might perhaps be well to 

 designate magnifications above the upper limit of 1,000 

 times the working aperture, as ''enlargements." Such 

 enlarged images are useful for drawing with the camera. 

 Their sharpness is a good test of the correct adjust- 

 ment of the objective. 



Submicroscopic Objects. — Objects below the limit of reso- 

 lution in size, but yet large enough, or opaque enough 

 (in a bright field), or bright enough (in a dark field), to 

 be visible as lines, eUipses, or discs, are submicroscopic. 

 Some objects, like small microbes, may be in size somewhat 

 above the limit of resolution, but invisible from not being 

 opaque. Viewing them dry would increase their opacity 

 by increasing their relative refrangibility. 



Mounting Media of Different Refractive Indices. — The 

 substance with which an unstained object is surrounded or 

 permeated is of importance for its easy visibiHty. The 

 most used of such media are probably; air, water, acetic 

 acid, lactic acid, glycerin, paraffin oil, immersion oil, 

 solid Canada balsam, styrax, monobromide of naphthalin, 

 "hyrax," and realgar. Of these, air, water and immersion 

 oil are perhaps to be preferred, for critical work, with air, 

 water or oil-immersion objectives, respectively; since they 

 do not affect the corrections of the objectives. Some 

 refractive indices are useful to be known, and hence the 

 following table has been compiled. 



