358 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
while that of the latter increases with the square of the square of the 
aperture. 
Experiment 8. — The apparatus was arranged as in experiment 6, but 
instead of an aerial image of bright lines and areas, an aerial image of 
dark lines and areas was photographed. The results in photos 19, 20, 
21, and 22 correspond with those in photos 1, 2, 3, and 5 respectively. 
Experiment 10. — A telescope was arranged as in experiment 2. In- 
stead of the paper diaphragm with a single slot, diaphragms with two- 
or more slots were used in the hood. Emitting points uncovered by 
pairs of slots (or an isolated zone of aperture) in a telescope behaved 
as we have seen the corresponding apertures of a Microscope objective 
behave in the projection of diffracted phenomena. 
Experiment 11. — Fine and closely ruled lines were observed while 
diaphragms with minute openings were held between the lines and the 
eye. The conditions were varied so as to convince one that the dioptric 
apparatus of the eye projects diffraction phenomena parallel with those 
j)reviously studied in images projected by the telescope, Microscope, and 
camera objectives. 
Experiment 12. — Photo 32 shows the lines of an Abbe test plate (the 
same shown inverted in photos 9 and 10), taken when the last emitting 
surface of the objective used was covered with a diaphragm which had 
an eccentric opening 1 mm. wide, and transmitted only such rays as had 
been previously diffracted in the plane of the object by the lines cut 
through a film of silver. Photo 33 was taken under the same conditions,, 
excepting that the eccentric opening was 2 mm. wide. Photo 34 was 
taken under the same conditions as photo 32, except that the slot 1 mm, 
wide transmitted central primary rays. Photo 35 was taken with a slot 
2 mm. wide, half central and half excentric, transmitting through its 
central half primary rays, and through its excentric half diffracted rays. 
Repetitions of the experiment show that, under parallel conditions, the 
same aperture gives the same resolution with either diffracted or primary 
rays. In other words, Aperture affects diffracted rays from an object as 
it does primary rays from an object. 
Dr. Mercer now gives at full length his reasons for considering that 
advantageous reduction in a cone of light between an object and the 
objective should not exceed, in the case of first-class objectives, one- 
fourth to one-third (never more than one-half) of the diameter of the 
cone. 
Experiment 13. — The general arrangement of apparatus was the same 
as in taking photo 1 (experiment 5), but instead of the card-holes an 
opaque card having a cross-shaped hole cut through it was placed 
against the bull’s-eye condenser. An aerial image of the cross was pro- 
jected in the plane of the Microscope stage by a 1 in. objective arranged 
as a substage condenser. This aerial image was then observed through 
a 1J in. objective and a 2 in. Huyghenian eye-piece. Let fig. 47 indi- 
cate diagrammatically the relative positions of the substage condenser, 
aerial image, and objective. The dotted line a shows the position of the 
first lens of the substage condenser, and b the position of its second lens. 
Let c show the position of the first lens of the objective, and d that of 
the final lens of the objective. Let 4 represent the Powell and Lealand 
substage diaphragm with circular opening “ 4 ” in use. Let the angular 
