Dreyer — On Adronomical Transit Observations. 509 



bright circle, partly on the dark border round it, where it was seen, 

 ■only by the field illumination in the telescope. The 3rd May, while 

 the eye-piece was too far from the wire, Hirsch remarked that the latter 

 was not seen as a straight line, but as a broken one, as the part which 

 was seen on the bright ground seemed to be moved out of its place to 

 the right. With adjusted eye-piece the line was seen straight ; with 

 a pushed-in one it appeared broken, and its central part moved to the 

 left. And this apparent shifting of the wires was found to be inde- 

 pendent of the placing of the lamp. 



At once awakened to this fact, Hirsch soon remarked that an 

 observer always would see two images of the wire if the eye-piece was 

 not exactly acljusted ; besides the principal one, which, fi'om want of 

 adjustment appeared diffused, a secondary, much fainter but sharper, 

 image was seen. The latter image seemed to occupy the real place of 

 the wire, and the relative position of the two images was reversed for 

 the two motions of the eye-piece. This gave, evidently, the explan- 

 ation of the principal fact, the acceleration or retarding of the 

 transits, caused by the abnormal position of the eye-piece ; and by 

 measuring the distance between the two images, by means of a mov- 

 able wire, this was found = 0-'"29, or nearly equal to the difference 

 a-d and a -p on page 507. 



This discovery, however, did not explain the observations made in 

 Zurich, nor those taken in jSTeuf chatel on the 2nd May. It had been 

 suspected that the position of the reflecting mirror in the telescope 

 might influence the phenomenon, and this was confirmed by an obser- 

 vation of Schmidt on the 4th May. According to this, a change in 

 the inclination of the reflector caused the secondary image to change 

 its position. While the reflector (whose incluiation could only be 

 changed a little) was in an extreme position, he could see the image 

 on the bright background of the meridian-mark, and the other one on 

 the comparatively darker field-background ; but on the latter he could 

 not see the secondary image, which Hirsch had seen on the preceding 

 ^ay. This image, however, became visible when he turned the re- 

 flector a little to the right ; it changed its place according to the 

 position of the eye-piece, but without coinciding perfectly with the 

 image projected on the bright back-gi'ound. After the reflector had 

 been turned still more towards the position in which it would give 

 most light, there came a moment when the two images in the part of 

 the field only illuminated by the reflector had the same intensity ; when 

 the reflector was turned still more in the same direction, the secondary 

 image disappeared again, while the principal one became more dis- 

 tinct. It was also found that the distance between the two images 

 ■decreased gradually, while the eye-piece was approached to the ad- 

 justment in which they coincide ; by moving it beyond the normal 

 position, the secondary image appeared again, but on the opposite 

 «ide. 



All these results were confirmed a few days later by MM, Planta- 



3 F2 



