VOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 333 



had lost much of the light necessary to show the extent of the cusps in their 

 full brilliancy. 



I do not give the calculations I have made of the extent of the twilight of 

 Venus, because my measures were not so satisfactory to myself as I wish them 

 to be ; nor so near the conjunction as we may hereafter obtain them ; neither 

 were they sufficiently repeated. My computations however, when compared to 

 those given in the paper on the atmosphere of Venus, show sufficiently that it 

 is of much greater extent, or refractive power, than has been computed in that 

 paper. Those calculations indeed are so full of inaccuracies, that it would be 

 necessary to go over them again, in order to compare them strictly with my own, 

 for which at present there is no leisure. 



I ought also to take notice here, that the same author, it seems, has taken 

 measures of the horns of Venus by an instrument which, in his publications, 

 he calls a projection table, and describes as his own ; of which however, those 

 who do not know its construction may have a very perfect idea, when they read 

 the descriptions of my lamp, disc, and periphery-micrometers, joined to what I 

 have mentioned above, of using the disc-micrometer without lamps when day- 

 light is sufficiently strong ; or even with an illumination in front, where the 

 object is bright enough to allow of it, such as the moon, &c. I remember 

 drawing the picture of a cottage by it, in the year 1776, which was at 3 or 4 

 miles distance ; and going afterwards to compare the parts of it with the build- 

 ing, found them very justly delineated. 



I have also many times had the honour of showing my friends the accuracy of 

 the method of applying one eye to the telescope, and the other to the projected 

 picture of the object in view ; by desiring them to make 2 points, with a pin, 

 on a card fixed up at a convenient place, where it might be viewed in my 

 telescope ; and this being done, I took the distance of these points from the 

 picture I saw projected, in a pair of proportional compasses, one side of which 

 was to the other as the distance of the object, divided by the distance of the 

 image, to the magnifying power of the telescope ; and giving the compasses to 

 my friends, they generally found that the proportional ends of them exactly 

 fitted the points they had made on the card. All which experiments are only so 

 many different ways of using the lamp-micrometer. 



As to the mountains in Venus, I may venture to say that no eye, which is not 

 considerably better than mine, cy: assisted by much better instruments, will ever 

 get a sight of them ; though, from the analogy that obtains between the only 2 

 planetary globes we can compare, (the moon and the earth) there is little doubt 

 but that this planet also has inequalities on its surface, which may be, for what 

 we can say to the contrary, very considerable. 



The real diameter of Venus, I should think, may be inferred with great con- 



