10 HOW TO WORK 



Any-sized aperture may be obtained, and the opening must be always 

 perfectly central. The graduating diaphragm made by Mr. Collins is, 

 however, the most useful diaphragm yet made, pi. XIII, fig. 59. 



DIFFERENT KINDS OF MICROSCOPES. 



15. students' Microscopes. Mr. Salmon (1853), Mr. Highley, 

 and Mr. Matthews were, as far as I know, the first makers in London 

 who brought out a really good, cheap, practical instrument, furnished 

 with foreign object-glasses. Two of Mr. Highley's microscopes are 

 represented in pi. Ill, figs. 1 1, 12 ; Mr. Salmon's student's microscope 

 is represented in pi. II. The microscopes of Mr. Ladd are thoroughly 

 well made ; and Messrs. Murray and Heath's new microscope (y/.) 

 is a very good and most convenient instrument. I would strongly 

 recommend all who are about to purchase a student's microscope 

 to examine the instruments of these makers, as well as the new 

 microscope of Messrs. Smith and Beck, which costs five pounds, 

 the students' microscopes made by Mr. Collins, of Titchfield Street, 

 and the very cheap instruments of Mr. Baker, Holborn. 



16. Large Microscopes. The large expensive microscopes are 

 provided with every instrument which modern science has placed 

 at the disposal of the observer. For delicate investigations many 

 of these are invaluable, but for ordinary work they are not necessary, 

 and their expense is so great as to place them beyond the reach of 

 the great majority of students. Very expensive and delicate instru- 

 ments are seldom necessary for ordinary work, and on those few 

 occasions when a very perfect instrument is required, the student 

 may appeal to some friend, who possesses a large microscope, for 

 permission to examine his object by it. The members of the Micro- 

 scopical Society have the advantage of using under certain regula- 

 tions most beautiful instruments provided with very high powers. 

 A very complete one has been liberally placed at the disposal 

 of the Society by Mr. Ross. These microscopes are now arranged 

 ready for work at the rooms used by the Society at King's College, 

 from 6 to 8 o'clock on each evening the Society meets. In the 

 Radcliffe Library at Oxford is placed one of Powell and Lealand's 

 large microscopes complete, including a T L, which may be used for 

 examination under certain restrictions. 



I should advise those who wish for a microscope as perfect as 

 can be made in the present day, to look at the beautiful microscopes 

 of Powell and Lealand, Ross, and Smith and Beck. In alluding 

 specially to these instruments, I wish it to be distinctly understood 

 that I do not in any way disparage the work of other and less cele- 



