Professor Barlow's New Achromatic Telescopes. 335 



Akt. XXV. — Notice respecting Professor Barlow's New 

 Achromatic Telescopes with Fluid Object-Glasses. 



As w^ have on various occasions directed the attention of our 

 readers to the great importance of the aplanatic fluid object- 

 glasses of our countryman Dr Blair, we are sure that they will 

 partake in our gratification when they learn that two gentlemen, 

 highly qualified both by their knowledge and their ingenuity, 

 have been for some time directing the whole energy of their 

 minds to this most interesting branch of practical science. 



Mr Blair, the only son of Dr Blair, has for more than two 

 years been busily engaged in constructing fluid object-glasses on 

 the principles discovered by his distinguished father. We had 

 an opportunity of looking through one of them, and though it 

 was only at a terrestrial object, yet it was easy to see its vast 

 superiority to all ordinary achromatic instruments. Professor 

 Barlow of Woolwich, to whom other branches of science owe 

 great obligations, has likewise been occupied with the same sub- 

 ject, both theoretically and practically. He has had completed 

 two telescopes, one of 3^ inches aperture, and another of 6 inches 

 aperture. With the former he can separate all the double stars 

 of that class which Sir William Herschel has pointed out as tests 

 of a good 3 J inch achromatic; and with the other he can of 

 course separate many closer double stars, but as it is only newly 

 finished he has not yet had any favourable nights for observa- 

 tion. 



The principle of construction of this telescope is different 

 from that of Dr Blair's, and possesses some important advan- 

 tages, as it increases the focal power of the instrument without 

 increasing the length of the tube; or, by keeping the focal 

 length the same, he can shorten the telescope very considerably, 

 viz. by one-third at least of the usual length. 



Professor Barlow will, we presume, submit these instruments 

 to the Board of Longitude, whose especial duty it either is, or 

 ought to be, to patronize with a liberal and active zeal every im- 

 provement on the telescope. If other nations have already 

 been allowed to outstrip ours in this branch of rival nlanufac- 



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