The 0.5m refracting telescope of the Van Vleck Observatory has been active in the determination of trigonometric parallaxes since its first observations in 1922. Its lenses were ground by C.A.R. Lundin of the Alvan Clark Co. for photographic use. Coma was minimized across the field and vignetting was also kept to a minimum. Partly as a consequence the focal curve is very steep in the blue and green regions of the spectrum, as is shown in Fig. 1. A Wratten No. 12 minus blue filter is used to filter out all wavelengths to the blue of about 5200 å. The region between 5200 å and 6000 å is very flat with the focal plane varying over a range of about one millimeter. Towards the red region it steepens, although not enough to impair images on photographic plates of emulsion types 103a-D and IIIa-F, the two in widespread use in recent years.