ON THE USE OF 



ing and perfecting their instruments. Those who can afford 

 one of Powell and Lealand's, Boss's, Dallmeyer's, or Smith 

 and Beck's, will in truth have all the assistance that art of man 

 can give to our natural eyesight. But for those who take up 

 natural history as a recreative science, and have not a large 

 sum at command, I would specially advise the choice of an 

 instrument in the first instance of such a make that the 

 various powers and appliances can be afterwards fitted as 

 the student advances in the study. 



The small binocular made by Messrs. H. and W. Crouch, 

 of Regent's Canal Docks, cannot be too highly praised, and 

 of which some short account may be acceptable. 



It had long been a desi- 

 derata with microscopists to 

 obtain the advantage of bino- 

 cular vision with stereoscopic 

 effect. Mr. P. H. Wenham was 

 the successful adapter of the 

 stereoscopic principle to the 

 microscope, which the follow- 

 ing extract from 'Recreative 

 Science' 1 * will clearly explain : 

 The result is obtained by 

 the introduction of a small, 

 but very accurately formed, 

 double reflecting prism, im- 

 mediately above the object- 

 glass, so as to intercept half 

 the rays of light which pass 

 through it. Eig. 1 will ex- 

 plain the principle : A is the 

 body of an ordinary micro- 

 scope ; at B a square hole is 

 cut, through which the prism 

 c is made to slide so far that 

 its edge will just reach the 

 FIG. 1. central line of the objective, 



* Recreative Science,' 3 vols. fcap. 4to, Groombridge and Sons, 

 London. 



