39. Camera lucida for drawing with the microscope, 

 designed by Dr. Abbe. 



E. 16. 1889. Made by G. Zeiss. Jena. 



The rays from tho paper and pencil are reflected first from the 

 large mirror in a horizontal direction, after which a small 

 mirror above the eye-piece directs them into the eye along with 

 the rays which have passed through the microscope. The ap- 

 pearance presented is that the pencil has been transferred to a 

 point just below the observer's eye. 



40. Slit and prisms used in the first form of spectrum 

 microscope. 



Lent by Dr. H. 0. Sorby, F.R.S. 1876. 



The slit is made of two flat strips of brass with even edges, 

 which can be approximated to each other and fixed parallel by 

 means of screws. This part of the apparatus is fixed into a 

 shutter erected in front of the microscope so as to exclude 

 extraneous light. 



To the substage tube of the microscope carrying an achromatic 

 condenser is fixed at its lower end a cap, in which is inserted, a 

 crown glass prism of suitable angle, so that the rays, which come 

 almost horizontally from the slit, are thrown up along the optical 

 axis of the instrument. Substances whose absorptive power it 

 is desired to test can be placed either in front of the slit or on 

 the stage of the microscope. This form of apparatus is now 

 quite superseded by No. 41. 



This apparatus is described in the exhibitor's paper in the 

 "Quarterly Journal of Science," 1865, vol. ii., p. 198. 



41. Micro-spectroscope, designed by Dr. H. C. Sorby. 

 E. 188. 1888. Made by J. Browning. 



The instrument fits into the microscope in place of the eye- 

 piece. In addition to the light coming through the microscope, 

 rays can also enter through the side of the tube and the two spectra 

 are viewed side by side for convenience in comparing them. 

 Both slits are provided with screws for opening and closing 

 them. The eye-piece has a rack and pinion for focussing. The 

 small cross tube at the top throws the image of a lozenge-shaped 

 spot of light into the field of view along with the spectra, and 

 it can be moved along by a micrometer screw so as to mark 

 accurately the position of any line in the spectrum. 



E. SLIDES. 



42. Holman's " current " slide. 



Lent by D. S Holman, Philadelphia. 1876. 

 This is a glass slide with two shallow circular depressions, 

 each of which has a deeper ovnl excavation at its outer side. 

 They a: e connected by a groove. 



