PHOTOGRAPHY TO THE MICROSCOPE. 267 



made by illumination to appear hexagonal. With a power 

 too low, I obtained photographs of the P. Balticum Avith 

 hexagonal markings ; with a higher poAver and larger angle of 

 aperture the tendency was to the spherical form. 



Fliotographs by polarized light. — A NicoFs prism is placed 

 under the stage, one also directly behind the objective; sun- 

 light is reflected from the mirror, and one of the prisms re- 

 volved till the field is dark ; with the low powers, by this 

 simple arrangement, photographs of objects may be obtained 

 which exhibit the structure revealed by the polarized light. 

 For higher powers it is necessary to use the polarizing arrange- 

 ment described by Von Mohl, 'Pogg.,' vol. cviii, p. 178, and 

 recommended by Carpenter ; that is, the light from a large 

 NicoFs prism is concentrated on the object by an achromatic 

 condenser. The perfection with Avliich this apparatus operates 

 may be inferred when I state that photographs of the cross 

 and rings in starch-granules as well as of the P. angulatum in 

 a dark field, were obtained by me without difficulty. Von 

 Mohl remarks that, with inferior apparatus, some very dis- 

 tinguished observers have been unable even to see these 

 appearances. Tlie selenite stage can, of course, be used 

 when it is found desirable. 



By arranging the apparatus according to the plan adopted by 

 Professor v. Kobellin his micro-stauroscope (Silliman^s Jour- 

 nal, [2] vol. xix, p. 425), the peculiar effects which microscopic 

 crystals produce on the cross and rings of calc spar, can be 

 photographed. By removing the condenser and objective, 

 as well as the slide containing the crystals, beautiful photo- 

 graphs can be obtained of the normal cross and rings ; the 

 systems of rings in other crystals can be photographed by 

 substituting them in place of the calc spar, as well as the 

 changes which they undergo by combination with jalates of 

 doubly refracting substances (circular analysis, &c.), it being 

 merely necessary to introduce the plates or films at the proper 

 positions. I was shown by Professor Dove some years ago, 

 while in Berlin, photographs of the normal cross and rings 

 around the axis of the calc spar, but, so far as I know, this is 

 the first attempt to photograph the changes which the cross 

 and rings undergo by the action of microscopic crystals. 



Stereoscopic photographs of microscopic objects can be ob- 

 tained with the monocular microscope, by covering, first, the 

 right half of the objective, then the left, by a suitable brass 

 cap, and taking two successive pictures. When using this 

 method it becomes necessary to move the mirror towards the 

 right or left hand with each saccessive exposure, which is not 

 only inconvenient, but often produces a slight distortion, that 



