CRYSTALS OF CARBONATE OF LIME IN THE EGG-SHELL OF REPTILES. 181 



way is, to adjust it down as far as it will allow, as we always make them 

 to stop at the corrected point. Bring the object into focus by the ad- 

 justment of the body, then adjust the object-glass till the upper surface 

 of the glass which covers the object is in focus ; this can very readily be 

 done while the person is observing, by taking between the finger and 

 thumb the milling on the object-glass, and turning it to the left ; then 

 bring the object again into focus by the body, and the adjustment is 

 perfect. For the principle of this important method of correcting object- 

 glasses for looking through glass, we are indebted to Mr. Andrew Ross. 



FIG. 10. Steel Disc, by which Drawings are 

 made. To use it, place the body of the Micro- 

 scope horizontally, take the cap off the eye-piece, 

 push on the disk to the line marked upon the 

 long eye-piece, and adjust it till the arm is hori- ]0 



zontal. 



FIGS. 11 & 12. Prisms for polarizing or 

 decomposing the Light. One (Fig. 11) is 

 adapted under the stage, in the same manner 

 as the achromatic condenser, the other in 

 the body of the Microscope, the draw of the 

 body is taken out, and the prism screwed 

 in, and the draw replaced. The polariza- 

 tion of light can be seen by turning round 

 the draw in the body, or the prism under Fig. 12. 

 the stage : the latter is generally preferred 

 Fig. ii. as being the most easy. 



[The price of the Instrument above described varies from forty to 

 sixty guineas, depending upon the number of powers and apparatus 

 attached thereto ; a very complete Microscope, however, of this kind 

 may be obtained for about forty-five guineas. Editor, .] 



XXXVII. ON THE EXISTENCE OF CRYSTALS OF CARBONATE OF 



LIME IN THE EGG-SHELL OF THE ALLIGATOR. 



By Dr. Henry Johnston, of Shrewsbury. 



IN the September number of this Journal (p. 108), it is stated, on the 

 authority of M. Turpin, that the particles of carbonate of lime which 

 give substance to the shells of birds and reptiles, are deposited in con- 

 fusion, molecule by molecule, and not in a crystalline form, as occurs in 

 the eggs of Cryptella and Helices. 



