MODEL STANDS FOR ACHROMATIC OBJECTIVES 



149 



lower end, serving as an amplifier, which was probably the first 

 application of a * Barlow lens ' to a microscope. 



Illumination for opaque objects was accomplished by a lenticular 

 prism, P, which was gimballed, and connected with a ring embracing 

 the body tube. 



We learn from Fresnel that the range of magnification was from 

 40 to 1,200 diameters. 

 The object-glasses were 

 composed either of two 

 doublet systems for low- 

 power work or of four 

 doublet systems all 

 screwed together for 

 high-power work, and 

 two oculars were pro- 

 vided of different power. 



It is interesting to 

 place one of the earliest 

 known English models 

 of the achromatic micro- 

 scope beside that of Sel- 

 ligue. It was made by 

 Tully the optician, of 

 London, who at Dr. Gor- 

 ing's instance had been 

 w r orking at the achroma- 

 tising of the microscope. 

 Selligue's is a manifest 

 modification of one of 

 the best forms as made 

 by Adams, Jones, or 

 Dollond. Tully made the 

 microscope figured in 

 116 from the working 

 drawings supplied by 

 Mr. J. J. Lister, who saw 

 that great accuracy of 

 workmanship and com- 

 plete steadiness in the 

 stand were needful for 

 achromatic microscopes, 

 and to this end they 

 adopted struts, such as 

 were used in telescopes, 

 connecting the body-tube with the base. The instrument is shown in 

 fig. 116. He also provided mechanical movements to the stage, but 

 no fine adjustment was applied. There was a sub-stage provided with 

 a rotating disc of graduated diaphragms. This microscope was made 

 in the year 1826 by Tully, but it was made from working drawings 

 supplied by Mr. J. J. Lister, who therefore is responsible for the entire 

 The sub-stage held a combination of lenses for a condenser. 



As compared with single lenses of equal power, from which so 



FIG. 116. Lister's achromatic microscope made by 

 Tully (1826). 



