176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
from the others. Telescopes 100, 150, and even 200 feet long were 
actually constructed, with lenses from 3 to 6 inches in diameter. 
These monstrous instruments, however, were too unwieldy for use, 
and the real work during the first century and a half after Galileo’s 
time was done with the smaller telescopes. 
Finally, in the middle of the eighteenth century, the color problem 
was solved by replacing the single-lens objective with a compound 
objective, each of whose separate components, made of different kinds 
of glass, canceled out most of the color effects of the other. These 
color-free (achromatic) lenses gave much better images and permitted 
the use of relatively short tubes for a lens of a given diameter. Tele- 
scopes immediately entered a new period of growth which culminated 
in the 40-inch lens, with a focal length of 63 feet, at the Yerkes 
Observatory in Wisconsin. The 40-inch was finished in 1892, and 
since that time developments have concerned lenses for special pur- 
poses rather than for greater light-gathering power. For technical 
reasons, it seems unlikely that larger lenses will be made in the 
foreseeable future. 
This greatest of all lenses had been ordered originally by a group of 
enthusiasts here in southern California in connection with a plan for 
a ‘University System.” The project did not fully materialize, and 
the unfinished telescope was bought and completed by the University 
of Chicago. 
The very large telescopes of recent times, in which light-gathering 
power is the most important consideration, are all reflectors, not 
refractors. The light is funneled to a focus, not by refraction through 
a convex lens but by reflection from a concave mirror. ‘These tele- 
scopes are free from color effects because all colors are reflected in 
the same way. 
The first reflector was made by Isaac Newton in 1672, in a deliberate 
effort to avoid the color troubles of single-lens refractors. His first 
model had a burnished metal mirror, about an inch in diameter, 
figured to a concave spherical surface, and mounted at the bottom 
of a tube about 6 inches long. The image, which would lie in the 
middle of the upper end of the tube, was thrown to the side by a small 
plane mirror set at 45° to the axis, just below the focus. Newton 
presented the toy to the Royal Society, where it may still be seen, 
sitting on a volume of his famous Principia. 
Although Newton’s reflector avoided the color problem, it suffered 
another defect, known as spherical aberration, arising from the 
spherical surface of the main mirror. It was not until 50 years later 
when Hadley, in 1722, found a method of parabolizing concave mirrors, 
that the development of reflectors finally got under way. About 90 
years ago metal mirrors were replaced by glass, silvered on the front 
surfaces. In our time aluminum has been substituted for silver, 
