G40 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fkj. 100. 



Ouc of tho Microsc()i)CS was exhibited at the Loan Collection of 

 Scientific Instruments in London in 187G. 



Joblot's Microscope. — In the same Museum referred to in the pre- 

 ceding note we also found the Microscope shown in fig. 100, which is 



constructed of ivory, tortoiscshell, 

 and brass. It bears no name, and 

 no record of its origin is contained 

 in the Museum. From the ornate 

 character and general resemblance 

 to a Microscope figured by Joblot * 

 we think it probable that ho was 

 tho maker. 



For the coarse-adjustment tho 

 socket slides on the pillar ; the 

 fine-adjustment is by means of a 

 screw passing down the pillar to 

 the stage-socket, and is actuated by 

 the shaped knob on the top. 



Hensoldt's Readings Micro- 

 scopes-t — Herr M. Hcnsoldt has 

 l^ublished an elaborate article " On 

 Reading Microscopes in general, 

 and on screw Microscopes, and the 

 scale Microscoi)es of the author in 

 particular." 



The great advantage of Micro- 

 scopes over verniers in reading 

 divided circles and scales, consists 

 in the greater magnifying power of 

 the Microscope as compared with 

 the lens of the vernier, as well as 

 in obviating parallax, and the possi- 

 ble excentricity of the latter. While 

 the lens only possesses a magnify- 

 ing power of 8-10 (and those of 

 greater power cannot well be em- 

 ployed), the Microscope can easily 

 be used with a power of 40-60, 

 which means a 5-7-fold increase of 

 efficiency. For if the interval be- 

 tween two divisions is increased 

 5-7-fold by optical means, the in- 

 termediate positions or subdivi- 

 sions can be estimated with greater 

 certainty in the same proportion. 

 With screw Microscopes in which 

 the subdivisions are measured by 

 the turns of the screw, the hundredth or sixtieth part of such a division 

 is determined with greater certainty in proportion as the magnitude of 



* ' Descriptions ot usaf^es de plusieur.s nouvcaux Microscoi)es, taut simples que 

 composez,' &c., par L. Joblot, Paris, 1718, fo]., pi. 14. 



t Central-Ztg. f. Opt. u. Mech., viii. (1887) pp. 242-G (3 figs.). 



