GHOSTS AND OCULARS. 57 



field and the brilliancy which accompanies the single lens. Its field 

 is nearly as big as that of the Mittenzwey eyepiece. I recently tried 

 out a \" ocular of this kind beside two Mittenzwey oculars and found 

 it distinctly superior to them in brilliancy and at least equal in defi- 

 nition of any eyepiece I have ever seen. It seems to be really a most 

 admirable form, particularly for small telescopes where it is necessary 

 to save all the light possible without sacrifice of other qualities. 



Finalh' I note in ^ 15 a modification of an ordinary Huyghen- 

 ian ocular which I have recently tried with reference to improving 

 the color correction. It is well kno-wTi of course that the objectives of 

 telescopes are usually over-corrected for color in order to compensate 

 for the under-achromatism of the eye combined with its eyepiece. 

 This over-achromatization can, of course, compensate accurately only 

 one particular degree of under-correction, that which corresponds to 

 son e selected power, or more precisely, to some selected aperture 

 of the eye, the achromatic error diminishing with the diameter of the 

 emergent pencil from the ocular. The point chosen is usually that 

 which corresponds to about .5 mm. diameter in the emergent pencil. 

 For all lower powers the objective is under-corrected, for higher powers 

 over-corrected. 



Therefore it will be of distinct advantage to have the lower oculars 

 somewhat over-corrected and the higher ones under-corrected. Now 

 it is easy to apply under-correction to a telescope ocular in practically 

 the same way that it is applied to the compensating ocular used with 

 apochromatic microscope objectives. Several years ago I had applied 

 the scheme in correcting the residual color in gun sights, and recently . 

 tried out the same principle by using, in a Huyghenian of 2:1 ratio, a 

 field lens of highly dispersing flint. The obvious result is to pull down 

 the focus for the blue end of the spectrum, initially too long on account 

 of over-correction. 



I have tested this particular ocular of 5 mm. equivalent focus on a 

 much over-corrected 3|^" telescope and a 3" of about the usual degree 

 of over-correction with the expected result of cutting down the blue 

 outstanding light, and reducing the in-focus image of an artificial star 

 to the normal hue of the secondary spectrum. The plan gi\'es every 

 appearance of working well, at least for objectives of moderate focal 

 length, and it would seem possible by following it out very materially 

 to improve the color corrections otherwise attainable. Since one 

 cannot achromatize for all powers the best plan would seem to be 

 to achromatize for the most generally useful power, and, since the 

 oculars are far the least expensive part of the telescope outfit, to 



