IOWA ACADEMY OF SCIENCE 
285 
is the diameter of the object which is to represent Mars. This value 
was so near to an even number that I made the object which I used 
2 cm. in diameter. We are told that Astronomers are able to detect 
certain so called “canals” upon Mars which vary from twenty to sixty 
or seventy miles in width. To represent these I used wires of suitable 
dimensions and placed them in such shapes that they formed patterns 
which could be studied in the different photographs which I later took. 3 
Fig.l. 
The apparatus in its final form is shown in diagram in Fig. 1 and 
in the photograph, Fig. 2. To the right and not shown in the picture 
is a large box containing an arc light. Light from this is passed through 
a lens in the front of the box and is focussed after reflection by a mirror 
onto the slit of the monochromatic illuminator which appears to- the 
right. From the illuminator green light of .000055 cm. wave length 
passes through a lens and is focussed upon a pin hole. This in turn acts 
as a final light source and illuminates the object which is mounted upon 
the optical bench. Directly in front of this object and on the same sup- 
port is a large acromatic lens which focusses the light from the point 
source upon the camera, aperture to the left of the figure. The camera 
in this case taking the place of a telescope. The aperture of the camera 
is separate from the camera itself and is mounted upon a support which 
has a micrometer screw attached. This permits a motion of the aper- 
ture and thus enables me to admit any portion of the diffraction pattern 
that I wish. 
To get the effects of a smaller sized telescope, all that is necessary, 
is to remove the 4 mm. aperture which is made in a plate of tin, and re- 
place it with a smaller opening. In my work a 4 mm. aperture rep- 
resents a 24 in. telescope; 3 mm., 18 in.; 2 mm., 12 in.; 1 mm. 6 in.‘; 
and .4 mm., 2.5 in. 
Fig. 3 is a series of photographs which show not only the different 
effects that we would get in looking at an object optically similar to 
Mars with telescopes of different sizes but also gives; an analysis of the 
image. Nos. 1-9 form a series taken with a 24 in. telescope. No. 1 was 
3 Because of a slight error in my calculations the apertures used are about seven 
per cent too small. 
