422 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



base. In reality the ' stiff and longish bristle ' is an extremely long 

 and delicate filament, totally unlike a bristle, being not tapered but 

 of nearly uniform thickness. The ' minute spines ' are in reality 

 very curious hairs, and, as far as we at present know, unlike any 

 others. They are delicate, lambent, bulbous hairs. What they 

 most resemble are the tentacles of a sea-anemone, and there are two 

 tubes discoverable which are important and comparatively large ob- 

 jects. There appears to be considerable probability that this inte- 

 resting object upon the last ring of the body of the flea, and known 

 as its ' pygidium,' acts as an auditory instrument. 1 In the examina- 

 tion of ordinary stained histological and pathological sections by 

 transmitted light, unless some very delicate point is sought, the con- 

 denser should have a stop, so that when the back of the objective is 

 examined the stop is seen cutting into the back of the objective by 

 about a third. This in some instances may be increased to a half by 

 diminishing the cone, but it is not advisable to use anything less 

 than a half unless it is absolutely necessary. As we have pointed 

 out above, high-class objectives will stand a j cone perfectly, and very 

 special objectives will bear even a.-J cone ; but for the ordinary run 

 of objectives will be found as much as they are able to bear some 

 indeed will not stand a ^ cone. Thus, to put it in round numbers, 

 an illuminating cone '2 N.A. is very suitable for ordinary work with 

 the apochromatic 1-inch and objectives, and one of '4 N.A. for the 

 ^ and ^, and one of '6 N.A. for the J and J. It is a good plan to 

 have one or two stops cut to give special cones, the N.A. of which 

 should be engraved on them. This subject is one of great import- 

 ance, as more than nine-tenths of all microscopic objects are 

 examined by means of transmitted light. 



Let us now note the effect of large cones on the simplest object. 

 A microscope is set up having an achromatic condenser with an iris 

 diaphragm ; let three good wide-angled objectives be chosen, say 

 1-inch, a ^-inch, and J-inch dry. Let the object be the one we have 

 already studied to some extent in this relation, viz. one of the stiff 

 hairs on the maxillary palpus of the blow-fly's tongue ; place the 

 1-inch on the nose-piece, open the full aperture of the condenser and 

 get the instrument into perfect adjustment. Now close the iris. The 

 haii- will be surrounded by a luminous border, which will give it 

 a glazy appearance, and its fine point will be blurred out. NOW T 

 open the iris until the last trace of that glaziness disappears. The 

 hair will appear as a different object, its outline being perfectly clear 

 and sharp. If the eye-piece is removed, about two-thirds of the ob- 

 jective back will be full of light. NOW T , without disturbing any of 

 the adjustments, replace the 1-inch by the ^, and it will be found 

 that the glaziness or false light will have returned. Let the iris 

 be further opened until the last trace of it disappears ; now, on 

 examination of the back of the objective, two- thirds of it will be found 

 full of light, and so on with the |. We call the attention of the 

 student to these facts as having a direct bearing upon the question 

 of the comparative effects of large and small illuminating cones, and 



1 Micros. Journ. April 24, 1885 : ' Pygidium of Flea ' (E. M. Nelson). 



