226 PHILOSOPHICAL TRANSACTIONS. [aNNO 1668. 



occur no less remarkable observations than in the former, both as to the 

 parts in the head and those in the body ; concerning the small weight of the 

 brain of this fish compared to the weight of its body ; several little fishes found 

 in the stomach, untouched by any teeth ; the ureters, the ovarium and ovi- 

 duct ; where he digresses to show that the mulierum testes esse ovario analogos, 

 and refers for further proof of this to his intended treatise, De partium genita- 

 lium analogia. 



On Grinding Optic and Burning Glasses of Non- Spherical Figures. 

 By Francis Smethjvick, Esq. F. R. S. N" 33, p. 631. 



Mr. Smethwick produced before the society, Feb. 27, 1 667-8, as specimens 

 of his invention, a telescope, a reading-glass, and two burning-glasses. The 

 telescope was about four feet long, with four glasses, the three ocular ones, 

 piano convex, were of this newly invented not-spherical figure, and the fourth 

 a spherical object-glass. This being compared with a common, yet very good 

 telescope, longer than it by about four inches, and turned to several objects, 

 it was found to excel the other by taking in a greater angle, and representing 

 the objects more exactly in their respective proportions, and bearing a greater 

 aperture free from colours. 



The reading glass, of the same figure, being compared with a common sphe- 

 rical glass, far excelled it, by magnifying the letters to which it was applied up 

 to the very edges, and by shewing them distinctly from one brim, through the 

 centre, to the other, which the spherical glass came far short of. But this ef- 

 fect the new figured glass performed only on one of its sides, whereas the sphe- 

 rical glasses perform their effect nearly alike on both sides. 



Lastly, the two burning concaves, of this new invented figure, were, the one 

 of six inches diameter, its focus three inches distant from the centre ; the other 

 of the same diameter, but less concave, and its focus ten inches distant. These 

 when approached to a large candle lighted, somewhat warmed the faces of those 

 that were four or five feet distant at least, and when held to the fire burnt 

 gloves and garments at the distance of about three feet from the fire. 



At another time, in the presence of Dr. Seth Ward, the deeper of the two 

 concaves turned a piece of wood into flame in the space of ten seconds of time, 

 and the shallower in five seconds, and that in autumn, about nine o'clock in 

 the morning, and the weather gloomy. The deeper concave, when held to a 

 lucid body, would cast a light strong enough to read by at a considerable dis- 

 tance. Also, that exposing the same to a northern window, on which the sun 

 did not shine, or very little, he perceived that it would warm one's hand sensi- 



