THE PHOTO-MICROGKAPHIC CAMEEA 



133 



geometric slide principle, and does not depend for its 

 accuracy on very fine workmanship. On this principle a 

 very simple and inexpensive apparatus might easily be arranged, 

 and would answer almost as well. The first model apparatus 

 of this type Was built up entirely of Wood, the girders being 

 made in the form shown, and the results obtained Were entirely 

 satisfactory. 



The Writer cannot too strongly recommend anyone who 

 may contemplate building up an apparatus to his own ideas 



FIG. 48. Author's Photo-micrographic Apparatus. 



to study carefully, the principles of the geometric slide as 

 enunciated by Lord Kelvin. 1 It is only recently that this 

 method has been appreciated in the construction of scientific 

 instruments, and although it is admitted that for heavy machines 

 it might have disadvantages, for any purpose such as is now 

 being discussed, it far surpasses any other method of design or 

 construction. The accuracy in working of any apparatus so 

 built is far less dependent on perfect workmanship than in any 

 other design. 



Fig. 49 illustrates the geometric slide and the method by 

 which it may be made. Let B represent the baseboard of the 



1 Thomson and Tate, Treatise on Natural Philosophy, part i, p. 154. 



