The present invention relates to a method for obtaining lacrimal fluid by means of contact lenses, as well as to the use of suitable contact lenses in carrying out the method of the invention. 
Knowledge about the composition of the lacrimal fluid, i.e. of mostly unchanged lacrimal fluid as produced by the cornea of the eye during resting secretion, can be gained only through analytical study of lacrimal fluid extracted from the eye.  In practice, this knowledge has significant bearing on such matters as whether a patient should be advised to wear contact lenses, or what a particular patient's tolerance is for various types of contact lenses.  It is in fact impossible to prevent the inserted contact lens from affecting normal lacrimal circulation on the corneal surface.  Since the lacrimal fluid serves, among other things, as carrier of nutrients and active substances, which it supplies to the cornea, a disturbance of the lacrimal fluid's circulation can also lead to an undesirable impairment of the corneal metabolism with correspondingly unfavorable side effects.  Based on the characteristic composition of the lacrimal fluid, the experienced opthalmologist or optician is able to select from among the various types of contact lenses the one that the wearer can be expected to tolerate.  In the case of patients who have been wearing contact lenses for some time, it is possible, in addition, based on the composition of the lacrimal fluid, to draw certain conclusions regarding a threatening or already existing disturbance of the corneal metabolism. 
In order to be able to carry out such studies, one needs a method for the collection of lacrimal fluid that is as simple as possible but effective.  The methods known up to now for the collection of lacrimal fluid can practically not be carried out without irritating the eye.  In the Schirmer test, for example, a narrow strip of filter paper is laid on the cornea or in the inferior conjunctival fornix of the eye to determine the rate of tear production.  The person being tested does not find this procedure to be a pleasant one since it is impossible to insert the filter paper strips without causing a slight burning sensation in the eye.  The unavoidable eye irritation causes an increased production of lacrimal fluid, the composition of which differs decidedly, however, from that of normal, so-called resting secretion since most of the additionally produced lacrimal fluid is caused by a heightened stimulation of the main lacrimal gland whereby the proportion of the aqueous component rises simultaneously with a change in the tonicity and a shift of the pH level in the direction of basicity. 
Even a pipette cannot be used to draw lacrimal fluid from the inner canthus without eye irritation, no matter how carefully handled.  In this case, also, there is undesirably enhanced production of lacrimal fluid with the unavoidable attendant change in it composition. 
Other attempts to stimulate the production of lacrimal fluid, for example, the careful massaging of the eye or the use of certain stimuli such as the stimulus of freshly peeled onion, do not obviate the above problems either, not to mention the fact that such treatments are not exactly pleasant for the patient. 
Thus, these known methods can only obtain lacrimal fluid that, owing to the irritation of the eye, contains an excess of the aqueous component and whose composition therefore differs markedly from an undiluted lacrimal fluid, i.e. from a resting secretion.  It is thus not possible, with known methods, to obtain pure resting secretion in which the composition of the natural lacrimal fluid can be measured. 
The object of the present invention is therefore to create a method which avoids the drawbacks resulting from the known methods for obtaining lacrimal fluid and which, in addition, allows for a markedly higher yield of lacrimal fluid in one single operation.