240 MEMORANDA. 



been refracted before they reach the objective, either by the 

 glass in the first slide or by the glass and balsam combined in 

 the second : it being thus proved that however the rays may 

 have been refracted by the different media, and hovi^ever we may 

 reason from theory upon those refractions, the actual working 

 apertui'e of the objective remains, under all conditions, exactly 

 the same ; for if it were reduced by any of those refractions, 

 not one single ray of the pencil a a could ever reach the eye of 

 the observer at the upper part of the microscope. Indeed, I 

 have tried this experiment witii a set of lenses of l-12th of an 

 inch focus and 152° of aperture, and on removing those from 

 the instrument, and placing on it another set of the same 

 power, but of 148° of aperture, the field was unilluminated , 

 and the effect of the black ground immediately produced, at 

 once pointing out that if by any refraction the aperture of the 

 lens had been reduced only 2'' on each side of the perpen- 

 dicular, the. effect of each refraction would have been imme- 

 diately seen. With respect to the markings on the Diato- 

 maceaj, and the manner in which they are effected by balsam, 

 I think Dr. Robinson has forgotten that none of those minute 

 and beautiful forms are without some portion of colour ; and 

 although balsam makes objects more transparent, and con- 

 sequently appears to rob them of a part of their colour, it still 

 leaves sufficient even in the smaller forms of the Diatomaceae 

 to render both them and their markings perfectly visible. 

 Who would ever contend that the markings on the larger 

 Pinnularia are not much better seen in balsam than when tlie 

 object is mounted dry ? and, as I stated in my last communi- 

 cation, I have two slides of the N. rhoinhoides ('Amician test), 

 the one mounted dry and the other in balsam, and I can at all 

 times see the delicate markings on this object quite as well, 

 or even better, on the specimens in balsam than on those 

 which are mounted dry. To an uneducated eye the markings 

 on the dry objects may appear more striking, on account of 

 their stronger colour ; but to a well-educated eye the superior 

 sharpness and exquisite beauty of those objects when balsam- 

 mounted is such as it would be vain to look for when they are 

 in their natural state. Again, I think it rather an unfair way 

 of testing the visibility of the markings, either in or out of 

 balsam, by the use of high eye-pieces ; for at the same time 

 that you reduce the light by the eye-piece, you destroy the 

 effect of the difference of colour, and therefore, of course, when 

 this difference is small, as it is in balsam-mounted Diato- 

 maceae, you might as well blot out tlie object altogether. To 

 see objects well when they are so very transparent, you want 

 all the light you can obtain, as the greater the light the 



