394 ME. W. HUGGINS ON THE SPECTEA OF SOME OF THE NEBULA. 



For the purpose of making these observations I had constructed, by Mr. G. Dollond, 

 an instrument in which are combined two forms of apparatus contrived by the Rev. W. R. 

 Dawes, F.R.S., and described by him under the names of "An aperture-diminishing 

 eyepiece," and "A Photometer of Neutral-tint Glass*." 



This instrument, which is adapted to the eye-end of my achromatic telescope, consists 

 essentially, first, of a diaphragm drilled with small holes moveable within the focus of 

 the telescope, which diminishes the aperture of the telescope in proportion as it is 

 advanced towards the object-glass ; and secondly, of two graduated wedges of neutral-tint 

 glass, which slide in front of the convex lenses with which the focal image is viewed. 

 The aperture in the diaphragm which was used in the following observations has a 

 diameter of 0'06 inch. The diaphragm is moved by a screw, and its position is read off 

 upon a scale divided into tenths of an inch. The photometer wedges are 4 inches in 

 length and divided into forty parts. 



The observations were made in the following way. 



The diaphragm was placed so that all the pencils from the object-glass passed through 

 the small aperture without diminution. The nebulae were viewed through the wedges 

 of neutral-tint glass. These were made to slide before the eye, until the exact part of 

 one of the wedges was found, at which the nebula was extinguished. 



On August 25, 1865, a night of more than usual clearness, several estimations were 

 made of each of the three nebulae, No. 4628. 1 H. IV., the annular nebula in Lyra, and 

 the Dumb-bell nebula. In each case the estimation applies to the brightest part of the 

 nebula. 



The source of light selected as a standard of comparison was a sperm candle of the 

 size known as " six to the pound." 



The rate of burning of this candle on three occasions was — 



August 26 . 35 minutes lost in weight 132 gi's. =157*8 grs. per hour. 



31 . 38 „ „ „ 142 grs. =160-8 grs. 



September 7 . 41 „ „ „ 148 grs. =157-8 grs. „ 



It was found necessary to diminish greatly the light of the candle in order to bring it 

 within the range of comparison afforded by the moveable diaphragm. 



For this purpose a thick plate, with parallel sides, of neutral-tint glass was placed 

 before the flame of the candle. An examination of the neutral-tint glass with a prism 

 showed that the absorptive power of the glass for all refrangibilities in the brighter 

 portions of the spectrum was very nearly uniform. The amount of diminution of the 

 light of the candle effected by the plate of neutral glass was measured by a BuusEif's 

 photometer. When the light passed through the neutral glass, the candle required to 

 be placed at a distance of 6-5 inches to cause" the ungreased central spot of the photo- 

 meter to disappear. Without the glass, the disappearance took place when the candle 

 * Monthly Notices, Eoyal Astronomical Society, vol. xxv. p. 229. 



