122 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



positions. If the crystal is doubly refracting and is strongly dichroic 

 wc may be sure that the colouring matter was formed along with the 

 crystal, bxit if it is not dichroic wc must conclude that the colour was 

 due to a change which took place after the formation of the crystal. 

 We may thus infer that the colour of the spiculo) of Gorgonia was 

 formed at the same time and was not introduced afterwards." 



Mayall's Mechanical Stage. — Mr. J. Mayall, jun., has improved 

 "Weuham's singlo-iilate mechanical stage, by dispensing with tlie plate 

 which from its thinness is liable to flexure. The slide is made to lie 

 on the surface of the rotating stage-plate, being held in a hinged 

 frame connected, by a sliding fitting, with the mechanical movements. 

 The object-carrier is shown in fig. 10 as applied to the stage ; the 



Fig. 10. 



curved ann on the right and the straigLt arm on the left are hinged 

 with sprung fittings and open like the blades of a pocket-knife, to 

 admit the slide. The inner edges of the frame are bevelled inwards 

 BO that the sprung arms press the slide in close contact with the 

 rotating stage-jdate. 



To obviate the inconvenien'^e due to any unevenncss of surface of 

 the glass slides, a narrow strip of paper may be gummed near the ends 

 of their under sui face. 



'J'he use of this carrier is equivalent to a corresponding reduction 

 in the thickness of the stage, and as the slide lies in contact with the 

 surface of the rotating stage-plate, the flexure that is found more or less 

 in every form of mechanical stage, acting by one or more superposed 

 plates, is obviated. 



