September 1, 1896.] 



KNOWLEDGE. 



199 



remains mysterious, but great advances have been made 

 in recent years in our acquaintance with these and allied 

 phenomena, and we may hope for still greater accessions 

 to our knowledge in the not distant future. 



MICROSCOPY. 



By A. B. Steele. 



IT is stated on some authority that magnifying lenses 

 were not in use till about the end of the sixteenth 

 century. It was known long before then, however, 

 that letters were enlarged when seen through a globe 

 filled with water, but it was thought that magnifi- 

 cation depended upon the nature of the water or of 

 transparent bodies, and not upon the lenticular form of 

 the glass. From the gradual deepening of curves, no 

 doubt, the idea originated of producing lenses of shorter 

 and shorter focus, until the combination of a convex lens 

 as an objective with a concave lens as an eyepiece, 

 distanced apart by the hands, led to the discovery of the 

 telescope. Its conversion into a microscope would im- 

 mediately follow, for, as Hersehel says, a telescope used for 

 viewing very near objects becomes a microscope. 



The first microscope would most likely be in the shape 

 of a hand lens, and this would soon be improved upon by 

 mounting the lens at one end of a tube with the object held 

 at the focus or attached to a piece of glass. For higher 

 powers some system of focussing would be applied, either 

 by sliding the object cover or the lens, and for still higher 

 powers globules of blown glass would be used. It is said 

 that Huyghens brought microscopes from Holland of 

 minute spheres of glass about the size of a grain of sand, 

 and one of the earUest experts in microscopical observations 

 is said to have constructed an instrument of blown glass 

 and used it in the discovery of minute forms of life. The 

 honour of having produced the first microscope consisting 

 of a combination of lenses is generally ascribed to a 

 spectacle maker named Jansen, in the small town of 

 iliddelburg, in Holland. The date can only be approxi- 

 mately given as shortly before or after the beginning of the 

 seventeenth century. One of .Jansen's microscopes was 

 found at Middelburg in 1850, and was exhibited at the 

 Loan Collection in London in 187G. It resembles one of 

 the present-day compound microscopes without the field 

 lens. At the time of its manufacture clear glass was 

 scarcely to be had in Holland, and microscopes were 

 constructed with lenses of rock crystal and designed to view 

 opaque objects by reflected light. 



In 1637 Descartes published a description of a simple 

 microscope which was a decided improvement upon the 

 form commonly used before his time. It consisted of a 

 simple lens mounted in a central aperture in a polished 

 concave metal reflector, and was practically the same as 

 the lens constructed a century later. He also designed a 

 machine for grinding and polishing lenses, nearly a quarter 

 of a century before practical men had their attention 

 drawn to this important subject. But about this time the 

 man whose fame as a microscopist is best known, and 

 who gave the first real impetus to microscopy, was llooke. 

 He was the first to employ diffused light instead of direct 

 sunlight. He introduced the field lens, and invented the ball 

 and socket movement in the construction of the microscope. 

 He was also the first to discover that a drop of water 

 placed on the front lens of the objective would allow more 

 light to pass than a dry lens could in the proportion of 

 unity to the refractive index of water. Such lenses were 

 subsequently known as immersion lenses. S3 distin- 

 guished a person was Hooke that Hersehel speaks of him 



"as the great contemporary and almost the rival of 

 Newton." It was not till nearly fifty years after his time 

 that microscopes were provided with mirrors. The early 

 system of focussing seems to have been effected by a 

 ' screw-barrel " arrangement acting on the object, which 

 was clipped between two plates and depressed from the 

 objective by a spiral spring ; and " screw-barrel micro- 

 scopes " — so named from the objective being mounted in a 

 little barrel — became common in the eighteenth century. 

 Many improvements in the screw-barrel system of focussing 

 were introduced, but all were more or less defective, until 

 a plan was discovered by which the image would remain 

 steadily in the field, and allow the object to be viewed 

 during the actuaL process of focussing. 



The invention of means to determine the exact magni- 

 fying power of any objective is due to Benjamin Martin, 

 who applied a screw micrometer to the eyepiece with fifty 

 threads to the inch, so that the precise number of diameters 

 could be stated. A ruler, divided into tenths of an inch, was 

 placed under the microscope so as to have a tenth in full 

 view on the image ; then by measuring it with the micro- 

 scope, and counting how many turns were made in so 

 doing, the number of turns divided by five showed how 

 much larger the image was than the object. Martin 

 made a large compound microscope for His Majesty 

 George III., which is now in the possession of the Royal 

 Microscopical Society. The instrument stood on the floor, 

 and was so large that the King could conveniently use the 

 eyepiece while sitting in his arm-chair. The compound 

 microscope of to-day, known as the " Continental model,"' 

 by which most of the scientific work of our time has been 

 performed, was evolved out of Martin's reflecting micro- 

 scope. In designing and executing microscopes Martin 

 excelled all others of his day. But his fame rests chiefly 

 on his being the first to construct an achromatic objective. 



The possibility of applying achromatism to the micro- 

 scope attracted the attention of men like Wollaston, 

 Hersehel, and Brewster, but, notwithstanding the re- 

 searches of these, its adoption made slow progress. In 

 fact, there is very little difTerence in the definition of the 

 image between a microscope made last century and one 

 made during the first quarter of this. It was achromatism 

 that gave the stimulus to the discovery of a more precise 

 means of focussing, and since its application in 1821 by 

 the French optician. Chevalier, the development of the 

 microscope has made greater progress than during the 

 whole former period of its existence. The neglect given 

 to fine adjustment can only be accounted for by there being 

 little or no original investigation done with the microscope, 

 and consequently there was no impetus given to its develop- 

 ment. The best designs of fine adjustment were first 

 devised by Englishmen. To Powell wo are indebted for a 

 system of focussing applied to the nosepiece. In Andrew 

 Ross' instruments the fine adjustment tube was raised and 

 depressed by means of a screw acting upon the end of a 

 lever of the second order, while in those of Powell and 

 Lealand the screw acted upon a lever of the first order. 

 The rival to these was the " Jackson system," the fine 

 adjustment of which was deficient in delicacy and precision, 

 chielly owing to the extreme shortness of the acting lever. 

 Lister's discovery of aplanatic foci led to still greater im- 

 provements in objectives, for his investigations were taken 

 advantage of by three of the then leading firms of opticians 

 in London, and very fine objectives were produced after 

 long and persistent experiment. 



The first really pr;ictical immersion lenses wore made 

 by Prof. Amici, of Modena, and were improved upon by 

 Hartnack and others, who succeeded in producing such 

 excellent objectives as to make it possible to resolve the 



