NATURAL PHILOSOPHY. 149 



The diatoms are especially beautiful; and no one can view, with- 

 out a sense of profound reverence and unspeakable emotion, the 

 elegant structure of Ai-achnoidiscus and Heliopelta ; of Surirella 

 aud Pinnularia. 



" On thus accomplishing the illumination of opaque objects under 

 the highest powers of the microscope, a powerful aid to investiga*- 

 tion is furnished, which, I doubt not, will be rightly appreciated. 



" Au inexperienced microscopist may find some difficulty at first, 

 but a few trials will ensure success ; and, when properly used, there 

 is no want of light with the one-twelfth or even one-sixteenth 

 with the B or C eye-piece." 



Mr. Charles Stodder exhibited before the Massachusetts Insti- 

 tute of Technology, in December, 18G6, a new illuminator of 

 opaque microscopic oly'ects under higli powers, the objective be- 

 ing its own condenser, — the invention of Mr. Tolles. 



The principal difficulty met with in passing a beam of light 

 down through the objective of a microscoj^e, and thus condensing 

 a strong light upon an opaque object is, in the case of high pow- 

 ers especially, the reflection back of a considerable portion by the 

 lenses of tlie objective. This causes fog and obscuration of the 

 image, though the object be well illuminated. This reflection 

 takes place principally at the interior front surface of the front 

 system. 



To obviate this difficulty, a small rectangular prism, immedi- 

 ately above the front- system, is so far inti'oduced into the side of 

 the objective mounting as to slightly encroach upon the extreme 

 margin of the upper surface of the coml)ination. When parallel 

 rays are reflected by this prism down through the marginal parts 

 of the front covered by it, they will have their focus much beyond 

 the place of the object. As a medium case, their distance of con- 

 vergence would be ten times the focal distance of the objective ; 

 consequently, a much greater poition of the whole light incident 

 upon the front system would be transmitted, and whatever amount 

 experienced reflection would be dissijiated by travelling back 

 through the objective in a path widely diiferent from that of the 

 visual pencil. 



Mr. Stodder also exhibited a small telescope, of seven-tenths 

 of an inch aperture, and magnifying thirteen diameters, equal to 

 any instrument of two-inch aperture and three or four feet long, 

 with which he had been able to compare it. With this instrument, 

 which can be carried in the waistcoat pocket, he had been able to 

 distinguish the satellites of Jupiter, and similar astronomic ob- 

 jects. This was also made by Mr. Tolles ; and, if his present 

 plans succeed, the cost of telescopes of large size will be dimin- 

 ished one-half by the great reduction in the size of the lenses. 



FOUCAULT'S SHEATHED OBJECTIVES FOR THE TELESCOPE, 



The concentration of the luminous rays of liglit at the focus of 



the telescope, when the sun is the object to be observed, renders 



observations very difficult and sometimes even dangerous. M. 



Leon Foucault has conceived the idea of utilizing the property 



13* 



