224 Professor Barlow on Achromatic Telescopes, tyc. 



ment on the telescope itself. Fearful in the beginning of ad- 

 vancing too far in opening the lenses, the first experiment was 

 made with the fluid at a very inconsiderable distance behind 

 the plate, and the quantity of uncorrected colour was very 

 great. I next tried a distance of 18 inches, and the uncor- 

 rected colour was considerably less than before, but still too 

 great. With this distance the experiment was witnessed by 

 Captain Kater. I next opened the lenses 24 inches, and at this 

 distance the experiment was witnessed by Professor Airy, who 

 still detected some uncorrected colour, which, however, is not 

 sensible to my eye till the telescope is applied with a high 

 power to Jupiter, Venus, or some bright star ; neither was this 

 defect felt in any sensible degree by Mr South or Captain 

 Beaufort, who were also present. 



From the gradual diminution of the outstanding purple by 

 opening the lenses from contact to 24 inches, I suspect with a 

 distance of 32 inches (which is perhaps the most I can venture 

 upon with a focus of 48 inches for my plate,) that the red 

 would be outstanding ; and if so, the proper point must be 

 between these limits. 



This, however, remains to be verified by experiment. Should 

 it be effected, we may enumerate the advantages of this tele- 

 scope as follow • — 



1. It renders us independent of flint glass. 



2. It enables us to increase the aperture of the telescope to 

 a very considerable extent. 



3. It gives us all the light, field, and focal power of a tele- 

 scope of one and a-half times at least, probably of twice the 

 length of the tube. 



4. It is presumed that further experiments may enable us to 

 find such a distance for the lenses as shall reduce what has 

 been termed the secondary spectrum (inseparable from the 

 usual construction,) either to zero or to an inconsiderable 

 amount. 



Note. — It may be proper to state, that, between the tempe- 

 rature of 31° in February and 84° in August, and again at 31° 

 in December or November, I found no appreciable difference 

 in the index noting the focal length of the telescope. The 



