Object-glasses and their Definition. 219 



I have been trying a number of experiments with various forms 

 and combinations of lenses in different positions between the eye- 

 piece and object-glass, but I may state that I do not profess to 

 have got exactly a similar arrangement to the noted searcher. I 

 cannot arrive at this without incurring the risk of maldng a spu- 

 rious imitation.* But, before entering upon the land of promise, it 

 will be as well to note something of what has already been done in 

 this direction. 



At the time when the first achromatics were introduced, Dr. 

 Goring attempted to obtain achromatism by various combinations 

 and forms of lenses, intended to act as correctives, placed at intervals 

 between the object-glass and eye-piece. He admits that he failed ; 

 nor was success possible, as the arrangement is faulty in its very- 

 principle. From an uncorrected lens or object-glass an immediate 

 divergence takes place between the most and least refrangible rays 

 or the violet and red end of the spectrum, the angular separation of 

 the two rays of course increases with distance, and at length any 

 form of concave lens will be quite incapable of re-combining them. 

 It is for this reason that small concave lenses of dense flint have 

 never been successful when placed half-way down a telescope, having 

 a single object-glass of crown. The reason for attempting this plan 

 was to avoid the expense and difficulty of obtaining large disks of 

 flint glass free from imperfection. Even an excess of thickness in 

 the back lenses of a microscope will show an appreciable amount of 

 uncorrected colour, and they are worked as thin as possible in order 

 to avoid this error ; for artificial achromatism can only be obtained 

 by the ahru^jt refraction between two adjoining surfaces. We cannot 

 imitate the perfection of nature in the eyes of animals, wherein 

 colour is. corrected by a gradual increase of density. Dr. Goring's 

 last achievement was a low-power form of microscope, a description 

 of which was not pubHshed till after his decease ; this he termed the 

 " Megaloscope." The instrument was arranged for the purpose of 

 obtaining a large and flat field ; the eye-piece was the ordinary 

 Huygenian, of low power, and the object-glass proper was composed 

 of two similar achromatics ; but the peciiliar feature consisted in a 

 thii'd achromatic of shorter focus than the others, set in a racked 

 tube. By separating this, or bringing it quite close to the others, 

 the power of the instrument could be instantly doubled. About the 

 same time Mr. Lister made a series of experiments in this direction, 

 which resulted in the well-known " erecting glass." This consists 

 of two single plano-convex lenses of 1^ and 2^ focus, with an 

 interval of three inches ; the shorter focus and flat sides are next 



* Should a demand render it worth while for any leading optician to construct 

 the searcher, for his own credit's sake he would do it properly. But how is this 

 possible, when no illustrative description has been given from which it can be 

 made? 



