HIE, W. HUGGINS ON THE SPECTEA OF SOME OF THE NEBULiE. 395 



was removed to a distance of 126 25 inches. The disappearance was viewed with a 

 small telescope. The numbers adopted are the mean of several observations, the close 

 accordance of which showed that the fixed light behind the screen had remained of 

 constant intensity. The ratio of the squares of the distances shows that the light of the 

 candle was reduced by the neutral-tint glass to the 3^ part of its original intensity. 



The candle, placed in a lantern and screened by the neutral-tint glass, was fixed on 

 the roof of a house at a distance of 440 yards from the observatory*. It was desirable 

 that the candle should be at some distance, in order that its image in the telescope 

 should be formed at nearly the same distance from the object-glass as the images of 

 celestial objects. Besides, it was convenient that the flame of the candle should appear 

 small, when viewed with the convex lens, magnifying on the telescope 101 diameters, 

 with which the nebulae had been observed. 



The image of the flame of the candle in the telescope was viewed through the same 

 parts of the wedges of neutral-tint glass at which the nebulae had been observed to 

 become invisible. By means of the moveable diaphragms, and also independently by 

 means of diaphragms placed before the object-glass, the apertures of the object-glass 

 were found at which the flame of the candle became extinguished at the parts of the 

 wedges at which the nebulae had been observed to disappear. 



Sept. 7, 1865. Jan. 19, 1866. 



Aperture corresponding to nebula in Lyra . . 2*09 inches. 2'00 inches. 



„ „ Dumb-bell nebula . . 1*06 inch. 1*20 inch. 



Nebula No. 4628. 1 H. IV. 4-0 inches. 



Taking into consideration the circumstances of the observations, I adopt for the 



Dumb-bell nebula an aperture of . . . I'lO inch. 

 Annular nebula in Lyra an aperture of . 2'00 inches. 

 Nebular No. 4628 an aperture of . . . 4-00 inches. 



The disappearance of the nebulae with the neutral-tint glass wedges had been observed 

 with the full aperture of the telescope (8 inches), therefore the ratios of the areas of the 

 aperture corresponding to the diameters of 1"10 inch, 2*00 inches, and 4-00 inches, to 

 the area of the full aperture of the telescope, will give the intensities of the nebulae in 

 terms of the candle screened with the neutral-tint glass. The results are — 



Dumb-bell nebula . . . . =-^ 

 Nebula in Lyra .... = iV 

 Nebula No. 4628 . . . . = i 



of the intensity of the obscured candle. ' 



The neutral-tint glass reduces the intensity of the candle to the g-fy part, therefore the 



* I gratefully acknowledge the assistance of my friend, Mr. S. B. Bjitcaii), F.R.A.8., ■who took the manage- 

 ment of the candle. 



