May 21, 18 74 J 



NATURE 



7 HE COMING TRANSIT OF VENUS* 

 V. 



T T is probable that the observations of contact will be 

 ^ very materially supported by additional observations 

 made with the double-image micrometer. This instru- 

 ment was devised many years ago by Sir George Airy.f 

 It is the most convenient eye-piece micrometer which can 

 be used for measuring the distance between a pair of 

 stars, or, as in the present case, between the limbs of the 

 sun and Venus. The peculiarity of Airy's double-image 

 micrometer consists in this, that one of the lenses forming 

 an ordinary terrestrial eye-piece is divided in two, like the 

 object-glass of a heliometer. The one half can be slid 

 past the other, and the amount of displacement accurately 

 measured by a divided circle, concentric with the screw 

 which gives this motion. When the halves of this lens 

 are relatively displaced, two images of the object are 

 seen, as in the heliometer. If the distance between a 

 pair of stars be the subject of measurement, the line of 

 separation of the half-lenses is made to coincide with the 

 line ioining the two stars. The screw is now turned in 



one direction, until tlie image of one star given by one 

 half of the lens coincides with the image of the other 

 star given by the other half of the lens. The amount of 

 displacement is now read off. The halves of the lens are 

 again brought to coincidence. The screw is now turned 

 in the opposite direction, and a similar observation mide. 

 Knowing the value of the divisions on the divided circle, 

 these two observations give us a means not only of de- 

 termining the distance between the two stars, but also of 

 fixing accurately the reading of the instrument when the 

 half-lenses are in coincidence. 



It is eisy to see that after the internal contact at in- 

 gress, and before the internal contact at egress, measure- 

 ments may thus be made of the distance of Venus from 

 the sun's limb, from which the true time of contact may 

 be deduced, just as in the Janssen photographic method. 



But, besides, this double-image micrometer gives a 

 means of estimating the true time of contact in a manner 

 which may possibly be one of very great accuracy indeed. 

 Consider the case of ingress two minutes before the time 

 of true contact. From this time up to the actual contact 

 the distance between the cusps, where the limbs of Venus 



and the sun mret, will diminish with very great rapidity, 

 liy turning the micrometer so that the line of junction of 

 the half-lenses is in a line with the points of these two 

 cusps, the distance between them may be very accurately 

 measured. The observation may be repeated a number 

 of times. The great rapidity with which these cusps 

 approach, with a very slight motion of the planet, makes 

 it probable that each of these observations will give the 

 means of determining very closely the true time of con- 

 tict. 



There are great difficulties connected with observations 

 of the sun at such low altitudes as are required for the 

 apphcation of De I'lsle's and other methods. These will 

 materially affect the definition of the cusps, and it is not 

 certain that the micrometer method will give results so 

 valuable as might have been anticipated. 



But even in the eye-observation of contact the low 

 altitude of the sun will be a serious drawback. This 

 difficulty has been fully recognised by the Astronomer 



' Contimied from p. jo. t Greenwich Observations, i 



Royal, and, with the assistance of Mr. .Simms, he has 

 devised an ingenious eye-piece, which is likely largely to 

 reduce the inconvenience.* The chief difficulty is, that 

 at such low altitudes not only are the rays of light enor- 

 mously refracted by the earth's atmosphere, but the 

 colours are actually dispersed, as with a prism. Hence 

 the definition cannot be perfect. The principle of the 

 new eye-piece consists in employing a hemispherical lens 

 for the one next the eye. The surface of this lens ne.xt 

 to the eye is plane ; and the lens can be moved, by means 

 of a screw and slight spring, in a socket which is a portion 

 of a sphere the sime radius as the lens. By turning the 

 screw, various inclinations can be given to the plane sur- 

 face next the eye. But the curvature of the other surface 

 remiins the same, though a different portion of it is used. 

 Tne practicil result, then, of such an inclination of the 

 lens in its socket is simply the introduction of a prism 

 whose angle can be so varied as to correct totally the 

 atmospheric dispersion. 



* Monthly Notices of the R.A.S. vol. xxx. p. 5S. 



