The head of the first tower telescope is shown in Fig. 94.[16] A is the cœlostat mirror proper 17 inches in diameter and 12 inches thick, B the secondary mirror 12¾ inches in the shorter axis of the ellipse, 22¼ inches in the longer, and also 12 inches thick. C is the 12 inch objective of 60 ft. focus, and D the focussing gear worked by a steel ribbon from below. [16] Contributions from the Solar Obs. #23, Hale, which should be seen for details. [Illustration: FIG. 94.—Head of 60-inch Tower Telescope.] This instrument being for solar research the mirrors are arranged for convenient working with the sun fairly low on either horizon where the definition is at its best, and can be shifted accordingly, to the same end as in the Snow telescope. There is also provision for shifting the objective laterally at a uniform rate from below, to provide for the use of the apparatus as spectro-heliograph. The tower is of the windmill type and proved to be fairly steady in spite of its height, high winds being rare on Mt. Wilson. The great thickness of the mirrors in the effort to escape distortion deserves notice. They actually proved to be too thick to give thermal conductivity sufficient to prevent distortion. [Illustration: FIG. 95.—Porter’s Polar Reflector.] In the later 150′ tower telescope the mirrors are relatively less thick, and a very interesting modification has been introduced in the tower, in that it consists of a lattice member for member within another exterior lattice, so that the open structure is retained, while each member that supports the optical parts is shielded from the wind and sudden temperature change by its corresponding outer sheath. Still another form of mounting to give the observer access to a fixed eyepiece under shelter is found in the ingenious polar reflector by Mr. Russell W. Porter of which an example with main mirror of 16 inches diameter and 15 ft. 6 inches focal length was erected by him a few years ago. Fig. 95 is entirely descriptive of the arrangement which from Mr. Porter’s account seems to have worked extremely well. The chief difficulty encountered was condensation of moisture on the mirrors, which in some climates is very difficult to prevent. [Illustration: FIG. 96.—Diagram of Hartness Turret Telescope.] It is interesting to note that Mr. Porter’s first plan was to use the instrument as a Herschelian with its focus thrown below the siderostat at _F′_, but the tilting of the mirror, which was worked at F/11.6, produced excessive astigmatism of the images, and the plan was abandoned in favor of the Newtonian form shown in the figure. At F/25 or thereabouts the earlier scheme would probably have succeeded well.