GV-34 PHOTOGRAMMETRIC ENGINEERING 
accomplished by using a double-slide plate carrier, permitting an accurate 
slow motion of the plate holder. A guide star is kept centered on the cross- 
wires of a small eyepiece attached to the plate carrier; the observer bears the 
greater part of the responsibility for the accurate guiding of the telescope that 
is necessary to obtain good photographic records. 
The apparent dimensions, even for the nearest and for the largest stars, 
expressed in their angular diameters, are so small (less than 0”.1) that no 
photographed star image bears any relation to the star's angular diameter. 
Any apparent size, as seen with the telescope, is caused partly by the star's 
diffraction image, partly by turbulence due to the atmosphere. The photo- 
graphic image is appreciably enlarged by photographic action in the emulsion, 
and increases with the brightness of the star. 
3. AT THE TELESCOPE 
In one group of long-focus problems the material consists of photographic 
plates on which the position of the star in which we are interested,—the “central” 
star, —is referred to a ‘background’ of three or more reference stars. The classic 
example is the parallax work done with the largest existing visual refractor 
(focal-length 19.37 meters, aperture 102 cm.)—at the Yerkes Observatory—at 
the beginning of the century. The necessary techniques of observing, measuring, 
and calculating were developed by Frank Schlesinger (1871-1943), who suc- 
ceeded in obtaining parallaxes with an accuracy not earlier achieved. His 
methods of long-focus photographic astrometry are basic and complete; only 
minor improvements remained possible. 
The annual or heliocentric parallax is defined as the angle under which an 
observer, located at the star, sees the unforeshortened radius of the Earth's 
orbit. In other words, the annual parallax is the semi-axis major of the small 
ellipse which the star seems to describe on the sky in the course of a year. 
Stellar parallaxes are now measured with an accuracy of ".01 or better, 
resulting in percentage errors of less than 5 per cent,—even as low as 1 per cent 
—for the parallaxes, and hence for the distances of the nearest stars. 
Then there is the study of relative positions of the components of double 
stars, first developed by Hertzsprung. Both Schlesinger and Hertzsprung had 
visual refractors; they used panchromatic emulsions and a yellow filter, thus 
eliminating the blue light, and obtaining sharp images in the color for which the 
objective was corrected. 
The present paper shall be limited to star positions measured on a back- 
ground of several reference stars. 
Magnitude (brightness) and color differ from star to star; it is of particular 
importance, therefore, to be aware of the effects on the photographed positions 
due to the magnitudes and colors of the stars. Residual guiding error is mini- 
mized by aiming at magnitude compensation between central and reference 
stars. This may be accomplished by reducing the brightness of the central 
star by means of a small rotating sector in front of the plate. Sector openings 
of less than about 2.597 (extinction 4 stellar magnitudes) lead to a slight increase 
in positional error, and generally should be avoided. ''Coarse" diffraction 
gratings, in front of the objective, may be used to provide fainter companion 
images, symmetrically placed on each side of the star images. Both sectors 
and gratings play an important role in reducing differences in the apparent 
sizes of star images, with a resulting increase in the ultimate accuracy of meas- 
urement. 
Color effects are primarily due to dispersion in our atmosphere, although 
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