162 MEMORANDA. 



of the cone of lig'it, at the under surface at d d, the apex 

 being taken from tlie upper side of a a, it will truly represent 

 the loss of aperture caused by the first refraction of the one 

 surface of the medium. 



The section of the cone of light at d d may be rendered 

 visible by breathing on the under surface of a a ', but for 

 accuracy 1 have used a thin film of bees'-wax, as this allows 

 the diameter to be marked with a needle-point with great 

 nicety. 



In giving this additional explanation I do not, in the 

 remotest degree, intend to insinuate, that Mr. Sollitt is not 

 perfectly familiar with the theoretical facts upon which this 

 method of measuring apertures is based ; but I entirely blame 

 the want of perspicuity, which prevented me from making 

 myself understood in the first instance 



Tliere is a remark contained in my paper, which subsequent 

 observation has induced me to recall, or at least to modify, 

 the very general interpretation to which it is liable. The 

 expression that I allude to was this : — " I have invariably 

 found, tliat when very difficult tests are mounted in balsam I 

 cannot discover the markings." In contradiction to this, I have 

 lately succeeded, in many instances, in bringing out the striae 

 on some, of what may be termed very difficult tests when in 

 balsam ; but I do not see that this is to overturn the fact, that 

 we are actually seeing such objects with a diminished aper- 

 ture. But, liowever, the subject requires an investigation 

 that I have not yet had time to devote to it. I have thought 

 this notice due from me, as I asserted that the statement made 

 by Professor Bailey, of his employing no other than balsam 

 mounted tests, required some further explanation I may here 

 mention, that it is my opinion, tliat if an aperture of 125^ is 

 reduced to 71° on an object in balsam, it will still have a 



