ORDINARY CONDENSER IMPROVED, &C. 313 



In the application of these facts in connection with the illumin- 

 ating pencil, it will readily be understood that while the marginal 

 rays are forming an image of the object, the central rays pass 

 through, and, entering the eye, tend to obscure the image by the 

 glare and overpowering effect which they produce. It will now be 

 most unmistakably clear that it is the central, or "glare" rays, 

 which have to be stopped out, not the marginal rays which 

 give the image ; hence it will be equally obvious that the 

 shutting out of these rays by diminishing the aperture through 

 which the light is admitted, either to or from the lens of 

 the condenser, is altogether ivrong in principle, and diametrically 

 opposite to the course which both theory and practice suggest ; 

 consequently all such ingenious contrivances, like the " iris " or 

 " contracting diaphragm," are misplaced ingenuity, from seeking a 

 remedy in the wrong direction. 



A consideration of the preceding facts long ago drew my atten- 

 tion to the possibility of obtaining a more successful arrangement, 

 and after a fair trial of all the various condensers, from the elaborate 

 " Gillett " down to the ordinary " kettledrum," and in combination 

 with all kinds of stops or diaphragms, I was ultimately led to the 

 conviction that the form represented in Fig. 1 (and which was 



Fig. 1. 



described in the M.M. Journal at the time*) gave by very far better 

 results than I had been able to obtain by any other means. The 

 middle projection effectually cut off the central rays, while the 

 indentation in the line admitting more light than the opposite side 

 was intended to compensate for the loss by the reflections in the 

 prism, &c. 



This arrangement continued in use until rjuite recently, when, in 

 manipulating with the Reflecting Illuminator, it was then discovered 



* Vol. ix. (1873), p. 57. 



