withjiuid object-glasses. 341 



six inches, but have been prevented from doing so by the want 

 of glass of a sufficient thickness. 



After what has been said, it will perhaps be asked how it 

 happens that the manufacture of s^uch valuable instruments 

 has not been prosecuted hitherto. A full answer to this ques- 

 tion it is perhaps not in my power to give ; but this much I am 

 able to state from certain experience, that it has not arisen from 

 any defect in the instruments themselves. I know that many 

 years and much painful perseverance were employed in bring- 

 ing the invention to perfection, for to perfection I trust it will 

 soon be known that it was brought ; and although it may be 

 thought surprising that exertion should cease just at the time 

 when all the difficulties had been surmounted, yet I am per- 

 suaded that it is far from impossible. The ardour of discovery 

 will then be gone, and may even pass into a degree of depres- 

 sion if the previous anxiety and interest have been great. Be- 

 sides that the kind of exertion requisite for establishing a manu- 

 facture is very different from that required for prosecuting an 

 interesting object of science, and the capacity for both may not 

 co-exist in the same individual ; I may likewise be allowed to 

 add that my father's attention was diverted from that object by 

 being subsequently engaged in rendering services not less im- 

 portant to the country. He had previously made experiments 

 by which he discovered the method of preserving lime juice at 

 sea for any length of time, and being afterwards appointed 

 first commissioner of the Board for the care of Sick and Hurt 

 seamen, he succeeded, by his urgent representations, in pre- 

 vaiUng upon the Admiralty to order a regular supply of lime 

 juice thus preserved for the use of his Majesty's ships, the 

 consequence of which has been the complete fulfilment of his 

 prediction at the time, viz. the entire expulsion of that destruc- 

 tive disease, the sea scurvy, from the British navy.* Dr Blair 

 has, however, never lost sight of the object of forming a manu- 

 facture of these instruments, and was sometime ago at the pains 

 to instruct me by actual practice in every particular connected 

 with their manufacture. And in order to remove all hesitation 

 in regard to their permanency, I may add that my father has 



* To, learn the extent of the benefit thus conferred it is only necessary 

 to read the account of Lord Anson's Voyage round Cape Horn. 



