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SUMMARY OF CURRENT RESEARCHES RELATING TO 
smaller, brighter, and better defined, with a dim, hazy, overlapping disc 
becoming evident. When the diameter was 3/8 in., both discs were 
brilliant and well separated, their relative sizes and distance apart 
approaching truth. When the diameter was 1/2 in., the picture was 
more brilliant, the larger disc tending to appear star-like with irradia- 
tion. With the full aperture of 2J in. irradiation was marked in both. 
During these observations thin concentric circles of light were glimpsed. 
Experiment 2. — Instrument and all the conditions same as in experi- 
ment 1, except that in the hood was fitted a piece of stiff black paper 
instead of the iris diaphragm, the circular piece of paper allowing no 
light to enter the objective except that which passed through a slot 
corresponding with one of its diameters. Thus the objective was made 
rectangular in shape, with a narrow aperture in one direction, and a 
long or wide aperture in the other. The discs seen through the tele- 
scope appeared stretched out, as it were, into lines always crossing at 
an angle of 90°, the diameter of the instrument corresponding with the 
slot. The width of each line was determined by the long aperture ; the 
length by the narrow aperture. A comparison of the width of each line 
with its length show r ed the comparative effect of the two apertures in 
contracting the diffraction pattern. 
Experiment 3. — At a distance of 3 ft. in front of a Microscope the 
same pinholes used in the first two experiments were arranged so as to 
allow only such light from a lamp flame as passed through them to reach 
the mirror of the Microscope. The light reaching the mirror was 
reflected through the substage condenser to an aerial image of the pin- 
hole projected by the condenser in the plane of the Microscope stage. 
"The aerial image of the pinholes was the object observed through the 
Microscope. Seen with a small diaphragm opening behind the objective, 
the pinholes appeared as two discs just touching one another. With 
larger openings the discs became smaller, more brilliant, and separated. 
The effects of varying aperture (varied by means of diaphragm openings 
behind the objective) in this experiment with the Microscope were the 
same as those seen in the first experiment, when aperture was varied by 
means of diaphragm openings in front of the telescope objective. 
Experiment 4. — The instrument and all the conditions were the same 
as in experiment 3, except that a slot corresponding to one diameter was 
used instead of the central opening in a diaphragm behind the objective. 
The aperture of the objective thus became rectangular in shape. The 
image of the pinholes was observed while the slot was turned so as to lie 
successively in all diameters of the instrument. The effects were the 
same as those seen when the corresponding experiment 2 was made with 
the telescope. 
Experiment 5. — The apparatus and its arrangement same as in ex- 
periment 3, with two exceptions. First, for the two pinholes three 
were substituted, and a group of three parallel slits 2 mm. apart ; 
second, for the diaphragms with a single opening at the back of the 
objective were substituted diaphragms with two slots. The substage 
condenser projected these as essentially self-luminous aerial dots and 
lines in the plane of the Microscope stage. The full aperture image is 
shown in photo 1, and is most like the original object. Projected with 
five isolated slots, the image shown in photo 5 differs more ; while with 
