298 Solar Observation. 



case has been given of one eye-piece alone, out of a set, pro- 

 ducing such a focus of vehement heat just at the front of the 

 screen-glass, as partially to fuse its surface in two minutes with 

 only three inches of aperture. A closer position is said to have 

 saved the glass ; and we must hope, without injury to the eye. 

 Other methods of subduing the heat have been adopted 

 with success. The elder Herschel made use of a filtered mix- 

 ture of ink and water, enclosed between parallel pieces of glass. 

 The late Mr. Cooper, of Markree Castle, Ireland, found that a 

 glass " drum " containing alum-water was so effectual that he 

 could employ his whole aperture of 13*3 inches,* using merely 

 dark spectacles to subdue the glare ; while, on the other hand, 

 during the great eclipse of 1851, Lassell found that the free 

 heat of only 2*55 inches, with a focus of 32*5 inches, broke the 

 dark glasses " with most alarming rapidity." To avoid risk of 

 this kind, he used the wise precaution of previously exposing 

 them to artificial heat. An ingenious helioscope, in part 

 suggested by Sir J. Herschel, but improved and actually con- 

 structed by Colonel Porro, in Paris, deserves especial mention. 

 It is a modification of the Newtonian telescope, in which metallic 

 specula are replaced by those of unsilvered glass. The large 

 concave mirror of course transmits all but a very small propor- 

 tion of the incident light ; the second reflection takes place at 

 the surface of a small plane mirror, or " flat," as it is technically 

 called, which stands at the angle of complete polarization of 

 light ; whilo a third reflection is produced from another similar 

 mirror connected with the eye-piece ; or the latter may be 

 furnished with a " Nicol prism •" by the rotation of either of 

 which arrangements round the axis of the cone of rays, the 

 light, already reduced to a very minute fraction,t may, as those 

 who are acquainted with the mysteries of polarization will 

 readily perceive, bo further diminished to any required degree, 

 and the employment of coloured glasses rendered needless, 

 even with considerable apertures. This beautiful device has 

 also Hi 1, merit of great comparative cheapness, but the disad- 

 vantage of being nearly useless for other than solar observa- 

 tions, and we have no sufficient information as to its accuracy 

 of definition; 



* This great instrument, twenty-fivo feet in focal length, was, as far as I know, 

 the largest specimen of tho workmanship of the French optician, Cauchoix. Its 

 |iurr]iaso by the late possessor was the unintentional means of increasing the 

 cliinrnMons of (lie great achromatic at l'oulkowa, as tho Czar Nicholas, on learning 

 ite magnitude, wan determined not to be outdone in a private observatory, and 

 altered Ih'h original order for one upon a larger scale. It was employed at Markree 

 chiefly in the formation of an extensive catalogue of starB, and was recently offered 

 for sale in consequence of tho proprietor's death. 



t This and similar values are so differently stated in different places, even by 

 high authorities, that I have not specified them. The question of" the amount of 

 light reflected at various angleB of incidence seems still open to inquiry. 



