386 HOW TO SEE WITH THE MICROSCOPE. 



of apparatus will not make any very severe demands on your 

 mechanical skill. 



Those of my hearers who read the London Monthly Micro- 

 scopical Journal will remember that the capacity to display the 

 markings of Navicula angulata by central light, was considered 

 a feat only to be performed by low-angled glasses of superior 

 excellence. Some of you, too, may have read the report of a 

 certain biological committee who met in Philadelphia last year 

 for the purpose of testing the object glasses of several makers ; 

 one of the tests used was the exhibition of the markings of this 

 same dry mounted Navicula angulata by central light. 



I was simply astounded when I read the report, in public 

 print, of this biological committee, and learned that in their 

 hands that Angulata had defeated a wide-angled American 

 l-10th, and I immediately repeated the experiment, using a 

 similar l-10th, by the same maker, and worked with a dia- 

 phragm plate perforated with an opening, say, l-200th of an inch 

 in diameter, and placed almost in contact with the under sur- 

 face of the mount. The result was amusing enough ! I instantly 

 had the markings " clearly and accurately " defined ; the prob- 

 lem was, indeed, to avoid seeing them I In fact nothing short 

 of sheer intention, or, what is worse, bad manipulation, could 

 have defeated the objective. I then repeated the experiment, 

 using this time a balsam mounted Angulata the !No. 11 of the 

 Moller plate and was almost instantly rewarded, as in the pre- 

 vious instance. I am prepared to again repeat this experiment 

 this evening should you so desire. 



Again, the converse of what has been said is also true, namely 

 in working with a wide-angled objective, and with.oblique light, 

 it is important to shut out the central pencils. About a year 

 ago, I devised a simple little instrument, which gave access only 

 to a narrow wedge of oblique light, and thus gave added force 

 to the definition of the objective and this "oblique dia- 

 phragm " was permanently fitted to the stage of my stand. 



The most efficient instrument of this nature, however, is 

 "Wenham's reflex illuminator," an ingenious accessory, and so 

 contrived as to shut out all rays less than 41 interior, which, of 

 course, has emergence at 90 into air. The reflex illuminator, 



