NEW CLASS OF APLANATIC TELESCOPES. 431 



on such a subject ; better to test any guess one may make, 

 by experiment, than to mislead by theory without sufficient 

 data, or to lessen the value of facts by connecting them with 

 erroneous hypotheses. 



ON APLANATIC TELESCOPES. 



Phil Mag., March 1867. 



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

 tingham, last August, I suggested ' oily or resinous substances, 

 such as castor-oil, Canada balsam, &c.,' as materials to be 

 used, in combination with glass lenses, to reduce or annihilate 

 the defect in the achromatic telescope arising from the irra- 

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

 specification of a patent of Mr. Wray has been published, in 

 which these, among other substances of similar optical cha- 

 racter, 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 publication 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 her- 

 metically 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. Cooke, the well-known 

 optician. Such publication may assist others in promoting 

 this important object. Though at work on the subject for 

 some time previously, the first published record on the sub- 

 ject I can lay my hand on is in the Monthly Astronomical 



