ksa is not the substitute of cli after the verb dṛś 'to see'.,
This is an exception to the sutra (III. 1.45.) enjoining क्स. The root दृश् will form its Aorists by (III. I. 57.) and will take अङ् and सिच्; as अदर्शत् or अद्राक्षीत् 'he saw'.,
न दृशः
Adhyāya 3 · Pāda 1 · Rule 47