Method of finding the Refractive Index of a Liquid, 467 



already pointed out that the darkness inside a drawer just 

 pulled out, if the operator is sitting at a table in front of a 

 window with a good light, is such that fine fibres can readily 

 be seen upon it as a background. No velvet or smoked surface 

 or artificial blackness of any kind is comparable with it. On 

 such backgrounds fine fibres are to all intents and purposes 

 invisible. What is in many respects preferable to the dark 

 background, at least in certain operations, is a plain looking- 

 glass lying on the table. Fibres resting upon it become 

 intensely brilliant and visible, provided the eye is so placed 

 as not to see the sky light itself reflected from the mirror. 

 One method of making the fibres very easily visible without 

 influencing their torsion, is to smoke them with burning 

 magnesium or arsenic. I do not suggest arsenic, but I men- 

 tion it because of the very beautiful effect I once observed, 

 after destroying all life in a small hot-house by burning a 

 large quantity of bengal fire in which orpiment is a con- 

 siderable constituent. All the spider-webs remained perfect 

 with the spiders in their places as though alive, and the webs 

 were of a dazzling white but perfect in form, undragged by 

 the weight of the white arsenic upon them, thus contrasting 

 strongly with the catenary distorted webs so much admired 

 in frosty weather. It was this observation that suggested the 

 magnesium smoking. 



These last few points hardly come directly under the title 

 of this paper, but I thought them worth adding as bearing 

 upon the successful design of apparatus in which the full 

 limit of delicacy and accuracy obtainable by the quartz fibre 

 may be obtained, and upon the practical details of its 

 treatment. 



XLV. A Method of finding the Refractive Index of a Liquid ; 

 applicable when the Liquid is not Homogeneous. By T. H. 

 Little wood, M.A.* 



Apparatus required. 



THE chief piece of apparatus required for the method is a 

 telescope with fixed wires in the eyepiece, arranged so as 

 to be capable of motion along a horizontal scale, without 

 changing its inclination to the vertical. The horizontal 

 motion can be measured either by a vernier or by a micro- 

 meter-screw. 



* Communicated by the Physical Society : read February 23, 1894. 



