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 J^avicula 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 s?me 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 ! 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, 



