Parliculars respecting the Pancralic Eye-Tuhe. 1S7 



over common magnifiers, in variety of eflfective magnifying i)ower, 

 convenience, and portability, is as 300 to 1." 



I 



« s Bootes, as observed on the 25th of May 1819, by 

 ^^^_ Mr. H. Browne, F.R.S. and myself, with an achro- 

 inatie telescope of 30 inches focus and 2--j-Vl'>s aperture, made, 

 by Mr. George Dollond, With 270 the two stars were as per- 

 fectly and as'distinctlv defined as in the diagram, without either 

 rings, or rays, See. around them. The blue colour of the smaller 

 star remarkably bright. 



" To observe this double star to the utmost advantage, (espe- 

 cially the colour of the small star,) the illuminafing power of the 

 telescope must be in a high proportion to the magnifying power. 



■ " « Gennnornm, with 230, as represented in the dia- 

 gram : Castor does not retpiire much illuminating 

 power. I have shown it to several persons who did 

 not know that it was a double star, with one of Mr. Dojlond's 

 one foot portable telescopes, to which I applied a Vancratk Eye- 

 tube, which gave a power of /O times, and they described to me 

 its aj)pearance very accurately." 



[The above is an extract from Dr. Kitchiner's Letter to Sir 

 .Toseph Banks, P.R.S., which was read at the meeting on the 

 20th April 1820; and if this invention contributes to facilitate 

 astronomical researches, let it be enrolled among the many obli- 

 gations which science owes to the discernment of that indefati- 

 gable ])atron.] 



Another is made, which is adapted for terrestrial purposes, 

 magnifying from 55 to 200 times. 



N.B. The apparent diameters, and the distances of double 

 stars from each other, vary very much, according to their di- 

 stance from the meridian, the different states of the atmosphere, 

 and the distinctness and magnifying power of the telescope. 



Eyes also differ in a very surprising manner in the size that 

 these celestial objects ajipear to them. The same evening that, 

 with a i)owcr of ISO, the y)lanct Jupiter has appeared to me to 

 be al)ont an inch and a half diameter, a person, who observed it 

 tlie next minute, said it looked as big as the moon : auotiicr, 

 about four inches diameter; and a third thought it did not ap- 

 pear hirfjcr to his eye than a small pea. 



This Eye-tube is applied to the telescope in the same manner 

 Au2 ■'^ 



