THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MARCH 1867. 



XXIII. On Aplanatic Telescopes. 

 ByW. R. Grove, Esq., F.R.S. $*.* 



IN my address as President of the British Association at Not- 

 tingham last August, I suggested " oily or resinous sub- 

 stances, such as castor-oil, Canada balsam, &c," as materials to 

 be used, in combination with glass lenses, to reduce or annihi- 

 late the defect in the achromatic telescope arising from the irra- 

 tionality of spectra. Since the delivery of that address, the spe- 

 cification of a patent of Mr. Wray has been published, in which 

 these, among other substances of similar optical character, the 

 principal one being oil of cassia, are named for the like purpose. 

 Although provisional protection was obtained previously, the 

 public could know nothing of the invention until the publica- 

 tion of the specification : this was shown to me by a friend 

 last week. 



In it the author does not, to my mind, get rid of the greatest 

 difficulty which is experienced in the use of such substances, — viz. 

 the want of permanency in the telescope, arising from the unequal 

 shrinking or drying of the substances when used between lenses 

 not having touching curves. He says that the glasses, when the 

 edges are chamfered, are hermetically sealed by the correcting 

 substance ; but I think he will find that this will not bear the 

 test of time. It seems to me desirable that I should publish 

 experiments I have made on this subject at different periods 

 during many years, and which I have communicated to several 

 persons, among whom I may name Dr. Frankland and Mr. 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 33. No. 222. March 1867. M 



