36 PRACTICAL TREATISE ON 



opticians regarded as impossible the construction of a good 

 achromatic microscope." Dr. Wollaston, too, was of the 

 same opinion that the compound would never rival the simple 

 microscope. 



In the year 1823 experiments were commenced in France 

 by M. Selligues, which were followed up by Frauenhofer, in 

 Munich, by Amici, in Modena, by M. Chevalier, in Paris, 

 and by the late Dr. Goring and Mr. Tulley, in London. To 

 M. Selligues we are indebted for the first plan of making an 

 object-glass composed of four achromatic compound lenses, 

 each consisting of two lenses. The focal length of each 

 object-glass was eighteen lines, its diameter six lines, and its 

 thickness in the centre six lines, the aperture only one line. 

 They could be used combined or separate. A microscope, 

 constructed on this principle by M. Chevalier, was presented 

 by M. Selligues to the Academie des Sciences, on the 5th of 

 April, 1824. In the same year, and without a knowledge of 

 what had been done on the Continent, the late Mr. Tulley, 

 at the instigation of Dr. Goring, constructed an achromatic 

 object-glass for a compoiind microscope of one-third of an 

 inch, focal length, composed of three lenses, and transmitting 

 a pencil of eighteen degrees : this was the first that had been 

 made in England, and it is due to Mr. Tulley to say, that as 

 regards accurate correction throughout the field, that glass 

 has not been excelled by any subsequent combination of three 

 single lenses. Mr. Tulley afterwards made a combination to 

 be placed in front of the first mentioned, which increased the 

 angle of the transmitted pencil to thirty-eight degrees, and 

 bore a power of three hundred diameters. Mr. Lister, who 

 was engaged with Mr. TuUey in perfecting the achro- 

 matic object-glass, finding that all the microscope stands 

 hitherto made were not sufficiently steady for the use of high 

 powers, directed his attention to the improvement of this part 

 of the instrument ; and, in order to carry out his views, he 

 employed Mr. James Smith, now one of our first opticians, to 

 execute a stand on the plan represented by fig. 21. This 

 instrument was finished by Mr. Smith, on the 30th of May, 

 1826, and was the first of the kind constructed in this country 



