LOCAL PARTICULARS OF THE TRANSIT OP VENUS, ISyi-. 81 



found sharper tliau anotlier, or perhaps quite sharp, and the 

 amount of motion necessary can be seen and made at once. The 

 moon answers well for this plan, and gives, of course, the true 

 focus of the telescope. 



PHENOMENA OF INGRESS AND EGRESS. 



In order to understand clearly the nature of the phenomena 

 presented at the ingress and egress of the planet Venus in transit, 

 it is necessary to remember that all bright objects, such as the 

 sun, stars, &c., when seen in a telescope present a lictitious 

 diameter, or, in other words, a bright object always looks larger 

 than it really is. The defect arises fi'om imperfections in the 

 telescope and in part from peculiarities of vision which cause us 

 to estimate the size of a bright object unfairly when it is placed 

 in comparison with a dark one of the same size. Mr. Stone, who 

 has explained this, and made such valuable use of the fact in 

 explaining the last transit of Yenus, estimates the enlargement 

 as one-sixtieth of the diameter of Venus, and he strongly urges that 

 observations of ingress and egress shoidd be mainly directed to the 

 determination of the time of these two phenomena which are to 

 be explained presently, Icnown as donhtful and certain contacts. 



The first external contact is not considered important, because 

 in all the best stations for observing the transit the sun is low 

 down when it takes place ; and the sun's limb, always an unsteady 

 seething line, is then so much so that it is considered impossible 

 to observe at all satisfactorily when the edge of the sun becomes 

 dented with that of Venus ; but if it is seen the time should be 

 carefully stated. As a help in this matter, the Astronomer-Royal 

 suggests that such coloured glasses should be used as will absorb 

 the ends of the spectrum, and leave the green predominant ; and 

 he further suggests that, for observing the sun at small altitudes, 

 great advantage may be derived from adding to the eye-lens a 

 small prism of which the base is towards the earth. It must be 

 equal in effect to the dispersion produced by the atmosphere. 

 This is greatest, of course, for objects near the horizon ; but in 

 the inverting telescope it is in the upper part of the field, hence 

 the prism is so placed as to have the opposite effect. His method 

 for introducing this prism, which must be variable to meet the 

 variable states of atmosphere, is exceedingly simple and beautiful. 

 The lens of the eye-piece nearest the eye is made movable, so 

 that it may take any position between its normal one A D 

 (figure 9) and A E ; where it will be seen EAD is in eftect a prism 

 introduced, which is variable at pleasure. As the details of con- 

 struction of this eye-piece are not given, I have added to the 

 figure just enough to convey to any one wishing to make one 

 an easy method of doing it. First, the eye lens must be taken 



F 



