ety ea aa of Immersion Lenses: 143 
pin. o/s...) = 02890666 sin’ 85° oO 
Log. sin. ¢’ .. = log. 0°890666 a 1°949715 
+ log. sin. 85°... + 9°884254 
Log. sin. ¢’ = sum : = 9°833969 
Diiterence in 5 = iiss log. sin, “43° 1 = 9°833919 
Difference = 0:000050 
Hence 135 ; 50 3: : 60": : 29" nearly. 
The value ¢' required is ‘disratie 48° 1’ 22”, and as ¢=50°, the 
deviation = ¢ — 9’ = 6° 58’ 88”, or 
6° 59’ 
nearly, as given in Table I., in the fourth column opposite 50° 
obliquity, page 24, July No., M. M. J. 
DESCRIPTION OF PLATE LX. 
ON THE COMPARATIVE REFRACTIONS AND ABERRATION OF RADIANT PENCILS IN 
Immersion LENSES AND CoMMON OBJECTIVES. 
Fig. 1 represents a single plano-convex front of an immersion 
and Lealand made in 1869. 
The particle Q is supposed to be immersed in Canada balsam in each figure, 
and protected with a glass cover 0’ 003” thick. 
Radiant rays from Q at angles of incidence 20° 30° 40°, &e., passing through 
the balsam and glass cover, are incident upon the under surface of the water film, 
are bent away from the axis Q A, are again deflected towards it, and pass through 
the substance of the plano-convex in the directions Q A and 
th of Powell 
16 
Q-0 x", 
os x ae. pointe a, a m4 pee ee 
te poids he axis in ze ae scendleciiewetanek ‘et 
Q-5 we delineable. 
Q-6 Ie 
In Fig. 2 the water film being withdrawn, the primary aberrations or inter- 
sections of the refracted rays now cut the axis AQ at much greater distances 
apart, as shown at 2’, 2, %1,%2 U3, Ly Ls, Xp, and the primary 4 aberrations to be 
corrected by the objective system is at least four times greater for the rays Q R’ 
or outer shell of the conical radiant pencil, than for a similar shell shown at Q R, 
Fig. 1. 
“The utmost aperture transmitted by an air film is also shown at Fig. 2 by the 
angle A’ Q' R’, and for the water lens at Fig. 1 by angle AQ R. The course of 
the rays as they deviate from the straight line during their passage through the 
various strata represented, is laid down from Table it calculated in the manner 
indicated in the appendix. 
The angle of total internal reflexion for a ray passing from glass into water, 
marked as “ limiting aperture” in the Plate, is drawn at 
62° 57’, 
and that for a ray passing from glass into air at 
41° 48’, 
The quantity of defining rays radiant from the bright particle Q or Q’ mani- 
festly transmitted, is much greater in the Fig. 1 than Fig. 2: whilst the initial 
aberrations to be finally corrected are very much less in amount, and of much 
easier correction or compensation in immersion than dry objectives, which doubtless 
explains the American and European celebrity of the former. 
