140 On the Optical Advantages en ane eto. 
Since the quantity of nascent rays or volume of the pencil 
emanating from the brilliant particle immersed in balsam varies 
as the square of the angle of total internal reflexion, the results will 
obtain the following form :— 
TABLE V. 
Ratio of Quantity 
REFRACTIONS. of Nascent Pencils Aperture. Annes Mote 
transmitted, ‘ 
> ° ! ° t 
Plate glass fo air... ... 1 83 36 41 48 
55 water... 2h 125 54 62 57 
Flint glass to turpentine .. 34 156 46 78 23 
Refractions from Cover through Quantity of Necessary Objective! Limiting Angle of 
Refracting Film. Defining Pencils. Aperture, Transmission. 
It should be observed here that two special conditions affect the 
objective definition of a brilliant immersed particle (almost all test 
objects are mounted in Canada balsam) :— 
I. The condition of the rays passing by the particle, glancing as 
it were past its surfaces and producing the phenomena of dif- 
fraction. 
II. The conditions and affections of the rays nascent from the 
particle itself acting, when brilliantly iuluminated, as a new 
origin of light, and producing a variety of emergent pencils, 
themselves affected by the refractive properties of the brilliant 
particle. 
In the first case, the definition is curiously modified by the 
illuminating pencils; a slight turn of the concave mirror, for 
instance, requiring frequently a new cover-adjustment of the screw- 
collar. And the same principle is recognized by advanced photo- 
graphers engaged in the instantaneous and enlarging process. Not 
only do the rays of the ground-glass plate illuminated by the sun 
affect the picture, but the rays proceeding from the miniature itself. 
This subtle distinction is of the last importance for understanding 
both the effect of nascent rays emanating from the brilliant particle, 
and also the condition of the illuminating rays themselves, pro- 
ceeding from the mirror or condenser. 
Col. Dr. Woodward very justly distinguishes between the inter- 
ference phenomena produced by oblique sunlight without a con- 
denser. A great change, however, in the combined effects of the 
nascent pencils and of diffraction (or glancing) pencils may be pro- 
duced even by the use of sunlight, provided the condenser haye not 
a larger aperture than 40°, and a stop be placed upon its front 
glass perforated with a minute orifice of from ; pth to ,1,th of an 
