78 LOCAL PAETICFLAES OV THE TRANSIT OF YENUS, 1874. 



stone pillars. In front of the object-glass and driven by clock- 

 work is a plane mirror arranged so as to throw the image of the 

 sun steadily into the long telescope. Similar means are provided 

 for taking the photos, but of course the enlarging lens is not re- 

 quired, and the shutter used in the other form of telescope is not 

 suitable, and is to be altered, but how is not yet decided. 



For all forms except the last it is essential for good work that 

 the telescopes should be mounted on good firm equatorial stands 

 adjusted to meridian and elevation of the pole, and driven by clock- 

 work (I have devised a very simple clock for this purpose, which 

 I cannot now describe, but which I shall be happy to show to 

 any one interested) . 



For making observations of the time of ingress and egress the 

 instruments first described are sufficient ; but for either appli- 

 cation of the direct method, other things are essential beside a 

 satisfactory equatorial telescope and movement. For a small error 

 in the adjustment of the polar axis, though of no consequence 

 for observing ingress or egress, is fatal to the value of micrometer 

 or photographic results. And first, the micrometer, which must, 

 in addition to the means of measuring distances to a fraction of 

 a second, have the means of reading angles of position to w thin 

 10 seconds ; it is very desirable, also, that the field should take in the 

 whole diameter of the sun, and the measures be made alternately 

 to the nearest north and south points of the limb of the sun ; the 

 sum of every pair of observations should then be equal to the 

 diameter of the sun, which will aff'ord a good test of their 

 accuracy. Moreover, the measures require to be made as nearly 

 as possible in the same part of the field, or have the part speci- 

 fied vrith each observation and the value of the micrometer 

 determined for all parts of it. 



The best method of finding the zero of position is by allowing 

 a star near the Equator to run along the position Hue or wire 

 and repeat the observation a number of times, reading the posi- 

 tion circle every time. A small well-defined sun spot or the edge 

 of the sun may be used, but the result requires a correction for 

 the sun's change in declination. As Venus is in efi^ect moving 

 in two directions, it will be necessary to make the measure of 

 distance and angle simultaneous, or an error will be introduced. 



The micrometer may be made of great use in observing ingress 

 or egress, by assisting the judgment as to the time of apparent in- 

 gress or egress, in the following way : — As soon as Venus is 

 bisected at ingress, set the micrometer lines to the diameter of 

 Venus, as it appears on the sun's limb, then turn it so that one 

 line becomes a tangent to the limb at the point where the planet 

 enters, and as soon as Venus reaches the other line apparent 

 internal contact will have taken place. 



