296 



Mr. Grubb communicated to the Academy an improve- 

 ment lately effected by him in the mode of illuminating objects 

 under examination in the compound microscope. 



After noticing the several improvements which have been 

 made within the last few years in the optical part of this in- 

 strument, Mr. Grubb remarked, that its mechanical arrange- 

 ments have not undergone corresponding improvements, and 

 are still defective in some important respects. Among these, 

 Mr. Grubb noticed the power of directing the illuminating 

 beam on the object at all angles of incidence and in all azi- 

 muths, and of registering the position at which peculiar effects 

 are obtained. He referred to the very ingenious method pro- 

 posed by his friend, Mr. Thomas F. Bergin, for remedying in 

 part this defect, and which consisted in the employment of a 

 combination of a rhombic prism with Mr. Shadbolt's parabolic 

 reflector. By means of this arrangement Mr. Bergin was able 

 to direct upon the object a beam of light at one fixed angle of 

 oblique incidence, but in all azimuths. Mr. Grubb then pro- 

 ceeded to describe his improvement, which gives the power of 

 directing the beam at any angle of incidence and in any azi- 

 muth, and also enables the operator to register its position at 

 any time, and consequently to restore it with perfect certainty. 



"I assume," said Mr. Grubb, "the existence of an illu- 

 minating beam, achromatized and adjustable as to the angle 

 of convergence, the shape of its section, and the position of its 

 focus, and I seek to effect the objects described above, first, by 

 moving this beam in a plane passing through the optical axis 

 round the focus of the instrument as its centre, and, secondly, 

 by rotating the object itself in a plane perpendicular to that axis 

 round the same point. As the optical axis in the instrument 

 I use is vertical, these planes are respectively vertical and hori- 

 zontal. 



" I have fulfilled the first condition by mounting a suit- 

 able illuminator on a vertical circular sector (nearly a complete 

 circumference) concentric with the focus ; this part of the ar- 



