ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1035 



thousand. Spectroscopy has become a recognized science, not so mucli 

 because of its revelations in regard to the nature of light, as on account of 

 the application of the spectroscope as an instrument to the study of the 

 physical properties of matter and of motion not only on the earth, but in 

 worlds other than our own. 



In discussing the question whether microscopy can be regarded as a 

 science, we must always bear in mind the fact that a science is only a con- 

 venient name for a groitp of similar laios of nature, and that the term is 

 properly aj)plicable not only to the development of these laws, but to their 

 application to the useful economies of life. Thus we have the science of 

 engineering, in which mathematical analysis is as much an essential part as 

 skill in mechanical construction. But this analysis would serve no useful 

 purpose if did not rest ultimately on facts of observation. 



The limitations which necessarily belong to a definition of physical 

 science are clearly expressed by Tate in bis most admirable treatise on 

 Heat. He says : ' Nothing can be learned as to the physical world save by 

 observation and experiment, or by mathematical deductions from data so ob- 

 tained.' Now the Microscope as an instrument of research stands un- 

 rivalled, not only in respect to the precision of the observations made with 

 its aid, but also in the universality of its application in furnishing what 

 Tate calls ' the data so obtained.' 



Each succeeding year witnesses an extension of tlie range of its applica- 

 tions. Within a few years, while retaining its claim as an essential factor 

 in scientific research, it has also become a very material aid in many 

 mechanical industries. It is a common impression that the Microscope is 

 too delicate an instrument to be used in the ordinary operations of me- 

 chanical construction, and that the apparent necessity of using transmitted 

 light for the purpose of illumination is an absolute barrier to any extended 

 employment of the instrument. The latter difficulty is entirely obviated by 

 the use of the opaque illuminator invented by Tolles, by wbicli a bright 

 metal surface can be examined with the utmost ease, while actual experience 

 has shown that it is by no means necessary that the instrument should be 

 mounted upon massive piers insulated from surrounding objects. 



I cannot more forcibly combat this impression than by referring to two 

 cases within my own experience. The ' Proceedings ' of the Society of 

 Mechanical Engineers for 1884 contains a description of a method of 

 cutting a screw in which each thread is made to correspond in pitch with 

 equal subdivisions of a standard yard traced upon a metal bar. The screw 

 for the engine constructed for Cornell University was made in this manner. 

 Prof. Anthony has shown that the maximum accumulated error of the screw 

 does not reach 2 mikrons for a limit of 20 inches, while the actual error at 

 any selected point will not reach 1 mikron. This screw was cut in the 

 manner indicated, in the third storey of a building occupied by machinery, 

 which produced a decided tremor in every room. It was only found neces- 

 sary to make the attachment of the Microscope to the compound rest of the 

 lathe very firm, and to brace the bed of the lathe very securely from the 

 floor. 



The writer was recently called upon to ' level up ' the bed of a very 

 heavy planer, having ways 18 ft. in length. Several days had already been 

 spent in securing as good an adjustment as could be obtained with the aid 

 of a spirit-level of special construction, A plank 22 ft. in length, 8 in. in 

 width, and 2 in. in thickness was set up edgewise beside the platen of the 

 jjlaner, but insulated from it. A groove 1/2 in. wide and 1/2 in. deep was 

 ploughed in the upper face of the plank, and after having stopped both 

 ends, the groove was filled with mercui-y. The surface of the mercury then 



