62 Humes Projection Microscope. [Sess. 



micro objects on large screens was in a very primitive state, 

 £lnd it is only lately that any real progress has been made. 

 One of the great difficulties in the way of successful projection 

 is the want of a very liigh illuminating power which is capable 

 of easy manipulation. Even the lime - light, as used in the 

 mixing burner, is insufficient for the higher objectives. In the 

 earlier forms of lantern-microscopes, very little attempt was 

 made to collect all the rays of light emitted from the radiant ; 

 and, as a rule, no other condenser except the ordinary double- 

 lantern condenser was used. Since then, however, more lenses 

 have been introduced in this and other forms of micro-lanterns 

 for the purpose of, as nearly as possible, concentrating the 

 whole of the effective rays on the object. For this purpose 

 some provision has also to be made so as to adapt to higher 

 and lower objectives of varying aperture. Another difficulty 

 lies in the want of objectives specially suited to this work. 

 The objectives which are used to-night have been picked out 

 of a large number which, however suitable for ordinary micro- 

 scopic work, are useless for projection. Many objectives which 

 yield excellent results in the microscope, produce only a very 

 small-defined circle in the centre of the screen, while the 

 greater part of the field is very blurred. *It is very difficult, 

 indeed, to find one which gives sharp definition all over. This 

 defect for lantern-work is fully recognised by micro-objective 

 ^'nakers, and several are experimenting with the view of bring- 

 ing out glasses suitable for projection. Among these are Zeiss 

 and Wray, the former having a place fitted up, and under the 

 direction of his son, for this special purpose. In general, the 

 objectives used are of low power. Those which I have found 

 best are a 2-inch Zeiss ; a 1-inch Zeiss ; a |-inch Crouch ; and 

 a :^-inch Wray. This is about the limit possible, as even in 

 the case of a |-inch some objectives touch the cover-glass be- 

 fore the object is in focus. With low-power objectives and a 

 moderately large screen the best results are obtained. In the 

 matter of slides suitable for projection, those are, as a rule, best 

 which present any good contrast or decided markings or feat- 

 ures. They do not necessarily require to be thin sections, since 

 in the case of some very thin objects the light may be so bright 

 that all detail will be lost, and only a white glare appear to the 

 observer. In the mechanical arrangements of the apparatus, 



