234 Transactions of the Boyal Microscopical Society, 
As a rule, English microscopists do not attempt to show delicate 
surface markings with glasses below Jths and Jths, unless—as in the 
case of certain ^ and i^ths, specially made, in answer to demand, by 
Mr. Wray and others for this purpose — they have angular apertures 
so large as to involve an almost complete sacrifice of penetration. 
Objectives that have been in great favour have had about the 
following proportions of angular aperture to assumed focal lengths : 
^ inch, 90° and upwards; y%tH 1^0° and upwards ; Jths, 140°; |ths, 
and higher powers, 170°, or more. If we allow for some exaggera- 
tions in these estimates taken from trade catalogues, we still find that 
angles quite inconsistent with a good working distance from the 
object to be viewed, and with a useful amount of penetrating power, 
have been imagined necessary for the exhibition of delicate surface 
markings. 
In using an objective of large angular aperture, the extent to 
which that aperture is brought into action depends upon the illumi- 
nation ; and if only parallel rays are sent through the object, or 
rays of slight divergence, only a portion of the aperture is 
employed. This enables us to ascertain approximately with any 
objective how much of its capacity for receiving oblique rays is 
necessary to enable it to show given surface markings. Stops may 
also be introduced to lessen the acting angle of the objective, and 
Dr. Pigott has employed the Iris diaphragm, made by Messrs. 
Beck, specially mounted for the purpose. Whatever means are 
employed, it will be found that the best large-angled objectives will 
show most lined and dotted objects of difficulty with less than their 
full angles, and those most perfect in their corrections wiU do this 
in many cases with direct light on a clear day. 
The old opinion upon this subject, and one still common, will 
be found repeated in the latest edition of the * Micrographic 
Dictionary,' in the article " Test Objects," which, like many others, 
has not been brought down to date. It is there stated : " Now, 
if we examine a valve of Gryrosigma by direct light, the minute 
structure will be invisible, however small, or large, the angular 
aperture may be, or however perfect the defining power." 
At the time of copying this passage the author of this paper 
has a microscope in his library, opposite a north window. The 
instrument has the sub-stage mirror turned on one side, quite out of 
the way, and is pointed like a telescope towards the clear sky. With 
Powell and Lealand's immersion ith, their last but one, a perfect 
definition is obtained of P. hif^ocam^us with the A eye-piece of 
Ptoss's series. Beck's ^th with less aperture shows it with same 
eye-piece rather better, because not so much drowned in light. A 
-^th by Zeiss, of Jena (his D), aperture according to catalogue 72°, 
but by measurement of Dr. Pigott and the writer, 68°, gives 
admirable definition ; a C (J), also by Zeiss, aperture 48° suffices 
