56 SCIENCE PROGRESS 



270 diameters. About the year 1740 Wilson introduced further 

 improvements. A mirror mounted on a foot was provided to 

 reflect the light into the instrument. The object was placed 

 between two glass slips, which were held between two small 

 brass screws ; a screw-thread enabled the object to be raised 

 or lowered. 



The invention of the compound microscope seems to have 

 been accidental. While arranging some glasses for spectacles, 

 Zacharias Janssen, an optician of Middelberg, noticed the 

 greater power which could be obtained by using certain 

 combinations of lenses. His instrument has since been 

 gradually improved. Bonanimi, in 1691, added numerous 

 appliances : he used three lenses — and also three tubes with 

 which he could obtain different magnifications. A distinct 

 advance was the use of a condensing lens to concentrate the 

 light on the object. 



A short time later Marshall, in England, used a series of 

 object-glasses, and for his eyepiece used smoked glass. The 

 purpose of this was to render the coloured edges of the object 

 less distinct ; the method appears to yield good results, 

 especially with lower powers. It is a little strange that this 

 method of producing achromatic images seems to have been 

 neglected by later workers. In 1791 a Dutchman, Beeldsnyder, 

 constructed an achromatic objective by using two convex 

 exterior lenses of crown glass with a biconcave of flint glass 

 between them. 



Since this time, the great advances that have been made, as 

 a result of the work of Le Chevalier, Ramsden, Abbe, and others, 

 lie in the construction of lens systems in which the distortion 

 due to spherical aberration and the colour effects due to chro- 

 matic aberration are eliminated ; in addition, the introduction 

 of immersion lenses by Amici, in 1855, must not be forgotten, 

 since it marks an important step in the improvement of the 

 microscope. 



It was only during the latter half of the last century, largely 

 owing to the work of Abbe, that the limit to the resolving power 

 of the microscope was clearly recognised and understood : that 

 there exists a limit beyond which it is impossible to push the 

 resolving power of the microscope. If two objects approach 

 within a certain limiting distance, it is vain to hope to be able 

 to separate them with the microscope under ordinary conditions ; 



