LOCAL rARTICULAES OF THE TRANSIT OF YENUS, 1874. 89 



because the motion of the slide is accelerated by the spring, and therefore 

 quicker at the end tlian at the beginning. As a possible method of overcoming 

 this difficulty, for which something must be contrived, he suggests that a thin 

 disc of cardboard, three feet in diameter, and mounted like a glass in an 

 electrical machine, and having on the same axis a grooved wheel six or eight 

 inches in diameter, and connected with the card-board disc by ratchet, the 

 three-feet disc has its axis level and parallel vrith the optical axis of the 

 tube, and has a slit placed radially in it, which will expose in passing the 

 whole sensitive plate every time the disc is turned ; therefore, one exposure 

 will be made during the passage of the slit, and the light will be cut off during 

 the other part of tlie turn. The exposure is to be made as follows : — A weight 

 is attached to the grooved wheel by a string of such a length that when the slit 

 is near the line at which it begins to make the exposure the weight rests on 

 the ground ; the weight is now wound up nearly one turn, or until the slit 

 comes nearly in a line of exposure on the other side, or past it, as it were. 

 Now, when an exposvire is wanted, the weight carries on the disc with a con- 

 tinually increasing speed until it touches the ground, when of course its action 

 ceases, and the disc goes on with its own momentum at a speed quite uniform, 

 except the slight retardation due to friction. 



"Note. — As to definition, this is not quite correct. If a telescope lens of 

 the ordinary kind is a good one, it will be found to have a very sharp actinic 

 focus, from one-half to three- quarters of an inch further from the object glass 

 than the visual focus." 



Note added 15t?i Octoher, 1873. — Since the remarks on enlarging lenses, 

 in page 7, were written, I have found by actual experiment that a better form 

 of enlarging combination is made as follows : — Take two simple 2^^'^^>i'0-convex 

 lenses of equal focal length, and place them with the convex sides towards 

 each other, and separated by a distance equal to rather more (one and one- 

 fourth) than the foc:il length. 



The diameter of these lenses should be about one and a half times the 

 diameter of the image of the sun formed by the object glass, and the focal 

 length about three or four inches. The advantages are, — first, that the 

 enlargement is effected without any distortion measurable by ordinary means ; 

 and, second, the lenses being simple, affect the actinic rays most, and so 

 correct to a great extent the faults of a telescopic lens applied to photography, 

 and so produce sharper pictures. 



H.O.R. 



