708 



SUMMABY OF CURRENT RESEARCHES RELATING TO 



surface lias not been ground or polished on a spherical concave 

 disk, but has been fashioned on a lapidary's wheel, or by some 

 method equally rude. The convex side is tolerably well polished, 

 and though uneven from the mode in which it has been ground, 

 it gives a tolerably distinct focus, at the distance of 4^ in. from 

 the plane side. There are about twelve cavities in the lens, that 

 have been opened during the process of grinding it : these cavi- 

 ties doubtless contained either naphtha, or the same fluid which is 

 discovered in topaz, quartz, and other minerals. As the lens does not 

 show the polarized rays at great obliquities, its plane surface must be 

 greatly inclined to the axis of the hexagonal prism of quartz, from 

 which it must have been taken. It is obvious, from the shape and 

 rude cutting of the lens, that it could not have been intended as an 

 ornament ; we are entitled, therefore, to consider it as intended to be 

 used as a lens, either for magnifying, or for concentrating the rays of 

 the sun, which it does, however, very imperfectly.' " * 



Lindsay's Microscope-t — This is represented in figs. 133 and 134, 

 a is the lens fixed in a concave speculum ; 6 the object-holder ; c is 



a handle to which a longer arm 

 Fig. 133. can be screwed, and which can 



b3 turned up beneath the Micro- 

 soope ; at d is a graduated scale 

 for roughly focusing the various 

 lenses belonging to the instru- 

 ment ; e, /, ^ is a lever on which 

 the observer can place the fore- 

 finger at e, and the thumb at /. 

 Since the lever turns on h, whilst 

 i moves in a slit in the plate of 

 the object-holder, the latter by 

 this means is adjusted at the re- 

 quii'ed distance from the lens. 



Janssen's Microscope.t — Pro- 

 fessor P. Harting, in the course 

 of an examination of some old 

 optical instruments found in Mid- 

 delburg in 1866, § and attributed 

 to Janssen, discovered a com- 

 pound Microscope, of which a 

 section is represented in fig. 185 

 (l-4th natural size). The two tubes holding the lenses a and & 

 are of tin roughly soldered together, and sliding in a somewhat wider 



* The shading of fig. 131 representing internal stria3, is too strong, suggesting 

 more opacity tlian really exists. 



t Berifht iiber die wissenschaftliclien Apparate auf der Londoner Internat. 

 Ansstellung im Jahre 1876 (Achenbach und Falk), 1878, Part 1, pp. 52-3 

 (2 figs.). 



X Ibid., p. 50 (1 fig.). See also p. 46. § Album dcr Natur, 18G7, p. 2G1. 



