A SILVER P]T1 GLASS TELESCOPE. 41 



the slit either opposite to or al)ove c. A cntcli at /" prevents tlie shutter recoiling. 

 The sensitive plate is put inside the box as usual in a plate-holder. When a photo- 

 graph is taken, the spring shutter is drawn up so that the lower nick in the edge 

 of the quadrant is enten^d by the pin r, and the inside of tlie camera obscured. 

 The front slide of the platehdlder is then removed in the usual manner, and the 

 solar image being brought into proper position by the aid of the telescope finder, 

 the trigger retaining e is touched, the shutter tlies past c, and the sensitive plate may 

 then be removed to be developed. 



To avoid the very short (wposure needed when a silvered mirror of 188 square^ 

 inches of surface is used, 1 liave taken many solar photographs with an unsilvered 

 mirror, which only reflects according to Bouguer 2| per cent, of the light falling 

 upon it, and should permit an exposure 37 times as long as the silvered mirror. 

 This is the first time that a plain glass mirror has been used for such a purpose, 

 although Sir John Herschel suggested it for observation many years ago. Ikit 

 eventually this application of the unsilvered mirror had to be abandoned. It has, 

 it is true, the advantage of reducing the light and heat, but I found that the moment 

 the glass was exposed to tlie Sun, it connnenced to change in figure, and alter in 

 focal length. This latter difficulty, which sometimes amounts to half an inch, 

 renders it well nigh impossible to find the focal plane, and retain it while taking 

 out the ground glass, and putting in the seiisitive plate. If the glass were supported 

 by a ring around the edge, and the back left more freely exposed to the air, the 

 difficulty would be lessened but not avoided, for a glass mirror can be raised to 120° 

 F. on a hot day b) putting it in the sunshine, though only resting on a few points. 

 Other means of reducing the light and heat, depending on the same principle, can 

 however be used. By replacing the silvered diagonal mirror with a black glass or 

 plain unsilvered surface, as suggested by Nasmyth, the trouble^ sensibly disappears. 



I have in this way secured not only macute and their penumbrse, but also have 

 obtained faculpc almost invisible to observation. On some occasions, too, the precipi- 

 tate-like or minute flocculent appearance on the Sun's disk was perceptible. 



It seems, however, that the b(^st means of acquiring fine results with solar photo- 

 graphy, would be to use the telescope as a Cassegranian, and produce an image so 

 much enlarged, that the exposure would not ha\e to be conducted with such rapidity. 

 Magnifying the image by an (nepiece would in a general way have the same result, 

 but in that case the photograpliic ad\antages of the reflector would be lost, and it 

 would be no better than an acliromatic. 



§ 4. THE OBSERVATORY. 



This section is divided into </, The Building ; />, The Dome ; and c. The Observer's 

 Chair. 



a. The, Bidlding. 



The Observatory is on the; to]) of a hill, 225 feet above low water mark, and is 

 in Latitiule 40° 59' 25" north, and Longitude 73° 52' 25" west from (in>enwich, 

 according to the determinations of the (^oast Survey. It is near the village of 

 Hastings-upon-lludson, and is about 20 miles iiortli of the city of New York. The 



6 July, 1864. 



