M 
^e 
REPORT OF COMMISSION V GV-35 
imperfect collimation of the objective may contribute its share also. Atmospheric 
refraction depends on the wave-length and on the zenith distance. The refrac- 
tion at zenith distance ( may be represented to a high degree of approximation 
by the formula (u—1) tan {, where y is the index of atmospheric refraction at 
the observer's location. We write the above relation as R tan {, where R, the 
atmospheric refraction at (—45? is the so-called refraction constant; this is 
tabulated below, together with the dispersion per 100 Á. At moderate zenith 
distances the refraction, and hence the dispersion, varies proportionally to the 
tangent of the zenith distance. 
Dispersion 
A R per 100 A 
4,000 A 61" .34 — 108" 
4,500 60.89 — 072 
5,000 60.58 — 050 
5,500 60.33 — 037 
6,000 60.19 — 028 
6,500 60.06 — .021 
7,000 59.96 — 017 
7,500 59.89 — 014 
8,000 59.83 — .011 
Except at the zenith, each star appears as a spectrum, whose blue end is 
closer to the zenith than the red end. For stars of different spectral types, the 
energy distribution is different; moreover, the spectra differ in brightness. For 
positional work, the spectral range should be reduced in order to have as nearly 
“monochromatic” images as possible. The approach to monochromatism is 
obtained by the triple combination of objective, filter, and emulsion. Sharp, 
round star images are obtained as long as the effective radiation is within 
Rayleigh's (visual) criterion for focal accuracy. According to the latter, all 
images obtained within 4f2\ of the focus of a theoretically perfect objective are 
equally good. In this expression X is the wave-length that corresponds to mini- 
mum focal-length and f is the ratio of focal-length to aperture; the same cri- 
terion holds closely for photographs as well. Generally, for long-focus refractors, 
Rayleigh's limit is less than one millimeter. 
4. PHOTOVISUAL TECHNIQUE 
Proper choice of filter and emulsion keeps the range of light close to the 
wave-length corresponding to the minimum focal-length of the color curve of 
the objective. The photographic position still depends on the residual energy 
distribution of the star's spectrum, as ''filtered" by the objective (transparency 
and color curve), filter and emulsion. The effective wave-lengths of these star 
images depend on the spectrum, and to some extent the magnitude; however, 
with proper choice of filter and emulsion this dependence may be reduced to 
a minimum. Even with the best possible spectral compensation, small dif- 
ferences in effective wave-length are likely to remain. 
We have noted the rapid decrease of atmospheric dispersion toward longer 
wave-lengths, which gives an advantage to the photographic technique as 
applied with visual refractors,—referred to as photovisual technique. Take, 
for example, the photovisual astrometric technique as employed with the 
Sproul visual refractor (Figure 3). The aperture of the achromatic objective is 
61 cm., the minimum focal-length is 1,093 cm. for 5,607. A minus-blue (No. 12) 
Wratten filter is used in contact with the 5X 7-inch plate, eliminating practically 
all radiation on the blue side of approximately 45,100. A suitable range of 
radiation is admitted to the photographic plate by using the Eastman G-type