142 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



this microscope are twenty-four in number ; they vary in focal 

 length from four inches to one-tenth of an inch ; ten of them are 

 supplied with Lieberkiihns. A small arm, capable of carry ing single 

 lenses, can be applied at T, and when turned over the stage the in- 

 strument becomes a single microscope ; there are four lenses suitable 

 for this purpose, their focal length varying from -^th to ^th of an 

 inch. The performance of all the lenses is excellent, and no pains 

 appear to have been spared in their construction. There are 

 numerous other pieces of accessory apparatus, all remarkable for the 

 beauty of their workmanship.' l 



Benj. Martin not only in this way greatly advanced the 

 mechanical arrangements of the microscope, but he improved the 

 optical part. He used a Huyghenian eye-piece on the telescope 

 formula, where the focus of the eye-lens was that of the field-lens 

 3, and the distance between them 2 ; but instead of employing a 

 single eye-lens he broke it up into two of equal foci, that nearest the 

 eye being a ' crossed ' lens, and the other a plano-convex, the steeper 

 convexities of these lenses being towards each other. In addition to this 

 lie placed at a short distance above the nose-piece an equi-convex lens 

 of 5J inches focus ; this acted as a back lens to all the objectives, 

 so that when an objective was changed it was really only the front 

 lens of a compound objective that was altered. 



Cuff designed and made a microscope, in 1744, which Baker 

 figured and described in his * Employment for the Microscope ' in 

 1753, which possessed several conveniences and improvements. Xot 

 the least of these is that which gives greater delicacy to the fine ad- 

 justment than is found in any preceding model. It was subse- 

 quently further improved by the addition of a cradle joint at the 

 bottom of the pillar by Adams. Cuff also designed a simple form of 

 micrometer. 



There were three designs of microscopes by George Adams, of 

 London, in 1746 and 1771, which have many points of interest, but 

 scarcely contribute enough of distinctive improvement to the modern 

 forms of the microscope to detain us long. That designed in 1771 is 

 figured in the Adams ' Micrographia Illustrata,' and is reproduced 

 in fig. 111. 



In this instrument Adams claims to have embodied a number of 

 improvements on all previous constructions. He applied ' two eye- 

 glasses at A, a third near B, and a fourth in the conical part between 

 B and C,' by which he increased ' the field of view and of light ; ' 

 draw-tubes were at A and B, by which these lenses could be separated 

 more or less, but the probability is very great that these were 

 simply copied from the improvements of a like kind devised by B. 

 Martin and described above. He also arranged the object-lenses, or 

 * buttons,' a and b, to be combined ; seven * buttons ' were provided, 

 ' also six silver specula [' Lieberkiihns '] highly polished, each having 

 a magnifier adapted to the focus of its concavity, one of which is 

 represented at e,' and the ' buttons ' could also be used with * any 

 one of these specula ' by means of the adapter, d. 



1 A Practical Treatise on the Use of the Microscope, 3rd. ed. London, 1855, 8vo, 

 pp. 25, 26. 



