286 
IOWA ACADEMY OF SCIENCE 
taken with the diffraction pattern directly in the center of the camera. 
The rest were taken by moving the aperture of the camera 1/2 mm. to 
one side each time. This means that the central bright spot! was not in 
the camera when Nos. 5-9 were taken. You will notice that the lines 
have changed from black to white and that some of the original single 
ones now appear double. This is simply a diffraction effect! that we 
would expect to get under these conditions. No. 1 was taken under the 
best possible conditions and is a better image than can be obtained in 
actual observations of Mars or of my substitute for Mars. I have 
chosen to call this an optimum image. Now if we had used diffused light 
instead of light from a point source we would have obtained the sum 
total of an infinite number of images like the ones shown in this series. 
To investigate this point I made a composite picture of the first nine by 
making all of these exposures in one place instead of moving the plate 
as I had done before. From this exposure I obtained No. 10, which you 
see is not as good as No. 1. Realizing, however, that I had over em- 
phasized the poorer images in this series by longer exposures, I made 
No. 11 which is also a composite picture of Nos. 1-9. In this case I ex- 
posed the plate one minute for each one of the nine different exposures. 
This picture is equivalent to one obtained with diffused light and is 
almost as good as No. 1. From this I drew the conclusion that under 
normal conditions one should be able to see such details as those of the 
dimensions I had chosen, upon the planet with a 24 in. telescope. 
Nos. 12-20 form another series taken under the same conditions as 
the first series, except that the equivalent of a 12 in. telescope was used. 
While in the second series the optimum is very good we see that the 
images rapidly fall off in distinctness and that where above we have 
five good images and four poor ones, we now have two good ones and 
seven poor ones. The total composite, No. 21, is very poor while No. 22, 
the normal composite, is fairly good. It is quite evident, however, that 
No. 22 is not as good as No. 11. 
Nos. 23-27 form a series of optima made with telescopes of 24 in., 18 
in., 12 in., 6 in., and 2.5 in. respectively. While these images are bet- 
ter than can be obtained in actual practice they do show an interesting 
point. It is quite evident that the larger telescopes are the more ef- 
ficient. 
Nos. 28-32 and 33-36 were taken with the above telescopes with dif- 
fused light from the back, and reflected light from the front, respectively. 
These images give us an idea of what we should expect in actual ob- 
servation work and they correspond to telescope apertures given for 
the images immediately above them, from Nos, 23-27. The main point 
