174 THE AMERICAN MONTHLY \3 



UD 



used, between the source of light and the microscope. Gen- 

 erally speaking, a light filter of picric acid sliould be used 

 for specimens stained dark red, or violet ; for light red 

 stains, a greenish-yellow one ; and for preparation stain- 

 ed with methylene blue, a dark orange-yellow filter is re- 

 commended. — Knowledge. 



Notes on Microscopy. 



F. SHILLINGTON SCALES, F.R.M.S. 



Opaque objects are illuminated by several methods. 

 The most frequent way is to focus the light directly upon 

 the object by means of a bull's-eye stand condenser, re- 

 membering that the flat of the condenser must be nearest 

 to the object and quite close to it, the focus being short. 

 If a lamp be used, it will be necessary to raise it well above 

 the stage. A better way is by the use of a "side silver 

 reflector," which is a small, 8ilver,parabolic mirror, placed 

 close to the object and reflecting the rays of light there- 

 on from a lamp placed quite near and about level with the 

 stage. Its management is soon learned. Perhaps the most 

 perfect means of illuminating opaque objects is by the 

 now but little used "Lieberkuhn." This is really a specu- 

 lum fitted above and around the objective, the light be- 

 ing thrown from beneath the object and reflected down 

 again upon it. Its disadvantages, and those which have 

 caused it to be largely disused, are that each objective 

 must be fitted with its own Lieberkuhn, and that the ob- 

 ject must be mounted not upon a black background, but in 

 such a way as to give an annular ring of illumination 

 round it. When we deal with mounting, we will point 

 out that the generally recommended method of mounting 

 opaque objects upon a black background is not only un- 

 necessary, but inconvenient. The writer invariably mounts 

 such objects in an ordinary cell, and puts under them a 

 plain slide upon which a disc of black paper has been fast- 

 ened. What more is needed ? 



