18 



of lens. The heat ray of lower intensity is conveyed to a focus 

 by a lens of rock salt, as rock salt absorbs little or no heat, 

 either dark or luminous, the calorific powers of the different 

 coloured rays can be best compared by using a prism of 

 this substance. The phosphorogenic ray for the refracting 

 telescope is best transmitted by a lens of quartz, they are all 

 however concentrated into foci by metallic reflectors. 



The forthcoming transit will be recorded by photography, 

 for which purpose preparations are being made in England, 

 America, France, Prussia, Eussia, Portugal, and many other 

 places. By this means the epoch of each photographic record 

 may be determined with accuracy, the time of the exposure 

 being from 1-50 to 1-100 of a second. The angle of position 

 of successive situations of the planet on the sun's disc, as shown 

 on the series of photographs, and the distances of the centres 

 of the planet and the sun, are data that determine the chord, 

 along which the transit has been observed to within O'l'''', and 

 an error of V^ in the measurement will give an error of only 

 0185" in deduced solar parallax. 



In the photographic method there is the possibility of a sys- 

 tematic distortion, either optical or mechanical, and where 

 the determination of a definite point is so refined as that which 

 will have to be of the solar parallax, any distortion may be 

 considered a serious defect. The observation is also uncertain 

 on account of irradiation, and being only momentary, if missed, 

 the record is irretrievably lost. 



The Americans propose to exclude the distortion error by 

 dispensing with the secondary magnifier, and employing a 

 lens of considerable focal length (40 feet) in order to obtain an 

 image lin. in diameter. This plan is also being adopted in 

 Lord Lindsay's preparations, in order to obtain an accurate 

 image of the sun at least 4in. in diameter formed by rays as 

 little oblique as possible being reflected into the telescope 

 horizontally by means of a heliostat. 



Some observers intend using the spectroscope, especially for 

 examining the dark ligament seen by some observers during 

 last transit of Venus. This, however, is not always the case 

 when first the opaque globe of Venus begins to touch the 

 bright limb of the sun. " I know," says Captain Smyth, "by 

 my own experience, it may be noted within a second of time.'* 

 The same remark is made by Sir David Brewster, in his edition 

 of Ferguson's astronomy, the times by observation of internal 

 contact can be observed with much greater accuracy than any 

 angular distance can be measured, and on this depends the 

 superiority of the method. 



Venus at that time is at her maximum size, being 61-236*, 



