36 



Experiments on Angular Ajperture. 



produced is taken into consideration, I am sure that microscopists 

 will be willing to spare a little more time and labour over tbeir 

 manipulations so as to procm-e a superior quality of specimens." 



Since this paper was written I have had much more experience 

 in the use of Dammar varnish in microscopy, and mostly in pre- 

 paring specimens of diatoms; but, aU things considered, I think 

 Canada balsam is the best material to use for that purpose. But 

 as a cement. Dammar ranks very high, and I have put up a pre- 

 paration of it for our principal dealer in microscopic objects and 

 requisites, Mr. Miller, and he has found it extremely serviceable in 

 the fastening together of glass as in constructing zoophyte troughs 

 and growing slides. So my fellow-member, Dr. Arnold, favours 

 very strongly its use as a cement in anatomical preparations. After 

 having used it for some time and experimented considerably with this 

 medium I consider that the reason why the first specimen I had was so 

 clear was that it was thick. Dammar of good quality dissolved in 

 coal-tar benzole and concentrated is very clear, otherwise it is milky 

 until it thickens on the slide. Canada balsam, and, in fact, almost 

 all solutions of resins in essential oils — i. e. varnishes — can be 

 readily bleached by a few days' exposure to the sun in a closely 

 stopped bottle. They are then much improved for use in micro- 

 scopy. 



VIII. — Experiments on Angular Aperture. By E. B. Tolles. 



The foUovring is given to illustrate the comparative available angle 

 of dry and immersion objectives. 



In the figure, " A " represents a plano-convex lens, nearly hemi- 

 spherical, applied centricaUy to an objective at its front face. The 

 objective used had an angle of over 170°. 



When the hemispherical lens is thus applied to the objective an 

 air space of course exists between the plane surfaces. On testing 

 the angle only 80° (or at most less than 82°) was obtainable. 

 Were the plano-convex removed, the angle indicated would be 170° 

 upwards. This was verified at the time carefully. 



