220 The Microscope. 



will produce a correspondingly good photograph of it. But for 

 powers less than \ inch, I would earnestly recommend those 

 specially corrected for photography; else sharply denned re- 

 sults cannot be depended upon with any certainty. I have 

 seen objectives of low powers, in which there was no apparent 

 difference between the Actinic and Visual focii, and which 

 gave — without any further corrections — negatives as sharp as 

 the image seen upon the focusing screen; but such instances 

 are rare, and cannot be counted upon. I would therefore re- 

 iterate — for all powers lower than i inch, employ only those 

 specially corrected for photography. 



The great majority of negatives are made with central illu- 

 mination, and for this purpose it is necessary to remove or turn 

 aside the mirror, and condense the light from the lamp upon 

 the object by means of an ordinary Bull's Eye Condensing 

 Lens, with the convex side turned toward the object. No fur- 

 ther condensation or modification of the light is necessary for 

 any below a ^th. For this and higher powers however, an 

 achromatic condenser of some form is absolutely necessary. An 

 ordinary object glass mounted in a suitable adapter, to fit the 

 sub stage will answer in many cases, a most excellent purpose ; 

 a Kellner or Orthoscopic Eyepiece, with a cap containing a 

 small perforation is still better, whilst a wide angle immersion 

 or an Abbe Condenser, is best of all. In all cases, working 

 with or without a condenser, a diaphragm for controlling 

 the volume and intensity of the light must be employed to ob- 

 tain uniformly excellent results. Different subjects require 

 widely different treatment in the way of illumination, with the 

 same objective. A full blaze of light with an open aperture 

 would be necessary in photographing the yellow chitinous por- 

 tions of an insect; whilst the delicate, colorless tracheal system 

 of the same, would necessitate the employment of a small top and 

 subdued light. In all cases, and especially with delicate trans- 

 parent tissues or other substances, all excess of illumination 

 must be carefully avoided, or the result will be a flat, spiritless 

 negative, void of contrast, and yielding prints of like indifferent 

 qualities. Experience alone, will enable one to successfully de- 

 termine the proper amount of illumination, but if this be made 

 a careful subject of study from the beginning, it can readily and 



