754 Transactions of the Society. 



XXIII. — On Tolles' Interior Illuminator for Opaque Objects. 



By William A. Eogees, F.R.M.S. 



(With Note by K. B. Tolles, F.R.M.S.) 



CRead 10th June, 1880.) 



The method of obtaining a sufficient illumination for opaque 

 objects by admitting the light, above the objective and reflecting 

 it down through the lenses upon the object, is due to Professor 

 Hamilton L. Smith, of Geneva, New York. ' 



It is described in a general way in the ' Annual of Scientific 

 Discovery ' for 1866-7, page 147, and is generally known as 

 the " vertical illuminator." The more recent modifications in the 

 form of its construction by Powell and Lealand, and by R. and 

 J. Beck, while adding perhaps a trifle to convenience in use, add 

 nothing new in principle. 



Two objections have been urged against this form of illumina- 

 tor : — 



First, That there is a great loss of light in the reflections from 

 the surfaces of the glass plate, and by the diminution of the 

 aperture in the case of the silvered mirror. 



Second, That observers generally find the successful manipula- 

 tion of it exceedingly difficult. 



The second of these objections may be overcome by attaching 

 the revolving mirror to an arm which receives its motion through 

 a ball-and-socket joint, attached to the outside of the tube, within 

 which the mirror revolves. The first objection is to a certain 

 extent obviated also by this device, since the mirror, being perfectly 

 under the control of the observer by means of the universal joint, 

 all the rays of light which are available can be directed upon the 

 surface to be examined. 



Nevertheless, even with the modification of the universal 

 movement of the mirror, this form of illuminator has not been 

 found well adapted to the requirements of the special problem 

 upon which the writer is engaged, viz. the comparison of standards 

 of length and the investigation of their errors of subdivision. In 

 the examination, for example, of two different metres, the illumina- 

 tion should be the same in kind, quality, and quantity for every 

 graduation examined. 



After having tried, as I supposed, every known form of illumina- 

 tion without success, I was dehghted to find in Carl's ' Repertorium 

 for Experimental Physics ' for 1877, what appeared to be a new 

 method of meeting the difficulties of the problem. In volume xiii. 

 page 566, Professor Wild describes a vertical comparator which 

 seems to meet in an admirable way all the difficulties which relate 



