226 The Microscope. 



accurate is the machine, that the first lines of the two sets, when 

 care is taken in making the adjustments, will be absolutely 

 concident, forming one and the same straight line. I have not, 

 in sets that I have examined, been able to detect the slightest 

 deviation, under any objective that would include the junction 

 of the two lines in its field. 



The comparison and investigation of any standard thus 

 ruled is next in order, but lack ol space will prevent any further 

 consideration of the subject in this paper. 



AN IMPERFECTION OF THE EYE AND TEST OBJECTS 

 FOR THE MICROSCOPE. 



BY LUCIEN HOWE, MEMB. ROYAL COLL. SURG., ENG. 



MY object in this short communication is to call attention to 

 the fact that fine parallel lines, whether drawn artificially 

 or existing in natural objects, do not make fair test objects for 

 the microscope. This is caused by an imperfection very com- 

 mon in our own eyes. In order to make that clear I would call 

 the attention of the society to the formation of the image of any 

 object on the retina of the eye. It is well known that this op- 

 tical instrument is constructed like the camera obscura. At the 

 anterior part there is a lens and at its focus a part upon which 

 the image is received. Theoretically, the lens should have a 

 perfect spherical surface, and therefore the image should be 

 equally well defined in all its parts. With the camera obscura 

 this is often the case. In the human eye, however, the lens has 

 its power augmented by a refracting surface, called the cornea, 

 which lies in front of it. When the type of perfection is reached 

 this cornea and lens have perfect spherical surfaces, and there- 

 fore the focus which they form upon the retina is equally dis- 

 tinct in every direction, vertically, obliquely, or horizontally. 

 Practically, however, this type of perfection is seldom or never 

 attained. The imperfect curvature is usually found in the cor 

 nea. Instead of being a section of a sphere it is more curved in 

 the vertical than in the horizontal meridian, and might be com- 

 pared, in a rough way, to the convex surface of a spoon, when 

 that is held horizontally. This variation from the typical form 

 can be shown by a number of simple experiments. 



