PRACTICAL TREATISE ON 



The microscope used by 



Hooke was a compound one 

 with three lenses, and is 

 ihown at fig. 1, and also 

 n the sixth figure of the 

 lirst plate of his work, in 

 vhich figure it wiU be 

 perceived that he likewise 

 ■epresents a method of iUu- 

 ninating opaque objects, 

 practised even at the pre- 

 sent day, the plan being to 

 place a globe of glass filled 

 with salt water or brine 

 immediately in front of the 

 lamp, the pencil of rays 

 from the globe is received by a small planoconvex lens, placed 

 with its convex side nearest the globe, by which the pencil is 

 condensed upon the object. Hooke also informs us of an 

 accurate method of finding the magnifying power of a com- 

 pound microscope, than which a better plan has not been 

 suggested in modern times, and as it would be difficult to 

 make his description shorter or more intelligible than it is, his 

 own words will here be transcribed : — " Having rectified the 

 microscope to see the desired object through it very distinctly, 

 at the same time that I look upon the object through the glass 

 with one eye, I look upon other objects at the same distance 

 with my other bare eye; by which means I am able, by the 

 help of a ruler divided into inches and small parts, and laid on 

 the pedestal of the microscope, to cast, as it were, the mag- 

 nified appearance of the object upon the ruler, and thereby 

 exactly to measure the diameter it appears of through the 

 glass, which being compared with the diameter it appears of 

 to the naked eye, will easily afford the quantity of its mag- 

 nifying." To Hooke also belongs the merit of having first 

 made globule lenses of high power, an invention which Hart- 

 soeker has also claimed; but if the dates of the works of 

 these respective authors be consulted, it will be seen that the 

 Micrographia of Hooke was published in the same year that 



