16 The Microscope. 



and good there is not the least doubt ; the question is in no way 

 open to discussion ; but that the condenser can be so used, and that 

 the objective will bear the strain, are matters which experience 

 shows cannot be accomplished without specially fine lenses, nor 

 without screens of the properly colored glass. Without the 

 latter, no eye can endure light of such intensity, and to get the 

 glass of the proper color to be placed between the source of 

 illumination and the condenser is not an easy matter. Mr. E. M. 

 Nelson failed to find the proper kind in England, and was com- 

 pelled to go to the continent for it, although Dr. Dallinger says 

 nothing about that. And it cannot, according to my experience, 

 be obtained in this country. The glass shouLl be, it is said, 

 cobalt blue pot metal, but as that can be found only in Europe, 

 and after a special search, what is the microscopist to do? 

 There seems nothing, except to lose some of the advantages of 

 a focused condenser, possibly all of the advantages of a critical 

 image, and to use the accessor}' racked down beyond the focus. 

 And none but the best objectives are sufficientl}^ well corrected 

 for spherical aberration to bear such a test ; with most lenses, 

 good lenses otherwise, the object under these conditions is lost 

 in a fog to be dissipated onh^ by reducing the illuminating cone 

 by diaphragms, or by racking down the condenser. What can 

 be done with the properl}^ colored screens and a focused con- 

 denser, I do not know, as the right kind of glass is not procur- 

 able, that to be obtained being either too dark, thus making the 

 light too dim, or not dark enough. Upon the use of these 

 screens and upon the proper color of the glass. Dr. Dallinger is 

 not sufficientl}' explicit. This is as important as are the instruc- 

 tions which he gives for the proper and ordinarily impracticable 

 use of the condenser. A suggestion from so great an authority 

 that the opticians should obtain and suppl}" the proper glass for 

 the modification of the light, so that an ordinary eye could bear 

 and utilize it, would probably have been followed by good 

 results, and microscopists would then be able to learn what good 

 objectives, although perhaps not apochromatic, would do un- 

 der such conditions. This is one of the minor defects in an 

 important section of the book. 



Referring to one of Bulloch's stands the editor says (italics 

 again his) : " It presents no special point, save the employment 

 of a Gillett condenser with the diaphraghm drum above the len- 



