104 



E. M. NELSON ON EVOLUTION OF THE MICROSCOPE. 



it is precisely similar to a Gregorian telescope; secondly, the 

 design is bad, because (a) the small mirror gets in the way of the 

 object, so that the object cannot be viewed unless it is placed at 

 a considerable distance from the large mirror, — the optical index, 

 therefore, of the instrument would be small ; (b) in order that the 

 instrument may be aplanatic, both mirrors require to have 

 ellipsoidal curves which are impracticable, and when spherical 

 curves are used the aberrations are additive. 



The next. Fig. 15, is a very efficient form of catoptric micro- 

 scope by Dr. Smith (1738), which is like a Cassegrainian telescope, 

 with this difference, that both mirrors are pierced with a hole 

 through their centres. Dr. Smith calculated four of these micro- 

 scopes, the main difference between them being in the residual 



Fig. 15. 



amount of spherical aberration they possessed (No. 3 having 

 the least). The calculations were for spherical curves; the 

 aberrations being subtractive the instrument is consequently far 

 superior in design to Barker's. He also describes the use of a 

 substage condenser, which had its aperture equal to that of the 

 objective mirror, the object being placed at their joint foci. 

 The foci of the mirrors were alike, and were 1 inch ; the power 

 of the microscope was 300 diams. ; the objective metal having 

 the large optical index of 32. This microscope was surpassed 

 neither theoretically nor practically for about eighty or ninety 

 years. 



We now come to Culpeper and Scarlet's microscope : this was 

 known as a " double reflecting microscope," and was first described 

 by Dr. Smith in his Compleat System of OjUicks, 1738. The only 

 point of importance to notice with respect to this model is that it 

 is the first instance where we meet with an illuminating mirror 



